More docs about mojom.

docs/intro/mojom_idl.md

 # Mojom IDL
 
 The Mojom IDL (interface definition language) is primarily used to describe
 *interfaces* to be used on [message pipes](message_pipes.md). Below, we describe
 practical aspects of the Mojom language. Elsewhere, we describe the [Mojom
 protocol](mojom_protocol.md). (**TODO(vtl)**: Also, serialization format?
 Versioning?)
 
 Text files written in Mojom IDL are given the `.mojom` extension by convention
 (and are usually referred to as Mojom/mojom/`.mojom` files). The Mojom bindings
 generator (**TODO(vtl)**: link?) may be used to generate code in a variety of
 languages (including C++, Dart, and Go) from a Mojom file. Such generated code
 "implements" the things specified in the Mojom file, in a way that's appropriate
 for the particular target language.
 
 ## Interfaces
 
 A Mojom *interface* is (typically) used to describe communication on a message
 pipe. Typically, message pipes are created with a particular interface in mind,
 with one endpoint designated the *client* (which sends *request* messages and
 receives *response* messages) and the other designed that *server* or *impl*
 (which receives request messages and sends response messages).
 
-For example, take the following Mojom interface definition:
+For example, take the following Mojom interface declaration:
 ```mojom
 interface MyInterface {
   Foo(int32 a, string b);
   Bar() => (bool x, uint32 y);
   Baz() => ();
 };
 ```
 This specifies a Mojom interface in which the client may send three types of
 messages, namely `Foo`, `Bar`, and `Baz` (see the note below about names in
 Mojom). The first does not have a response message defined, whereas the latter
@@ skipping 12 matching lines @@
 supposed to (eventually) respond by sending the response message. (Note: The
 client may include as part of the request message's metadata an identifier for
 the request; the response's metadata will then include this identifier, allowing
 it to match responses to requests.)
 
 The `Baz` request message also contains no data. It requires a response, also
 containing no data. Note that even though the response has no data, a response
 message must nonetheless be sent, functioning as an "ack". (Thus this is
 different from not having a response, as was the case for `Foo`.)
 
+## Structs
+
+Mojom defines a way of serializing data structures (with the Mojom IDL providing
+a way of specifying those data structures). A Mojom *struct* is the basic unit
+of serialization. As we saw above, messages are basically just structs, with a
+small amount of additional metadata.
+
+Here is a simple example of a struct declaration:
+```mojom
+struct MyStruct {
+  int32 a;
+  string b;
+};
+```
+We will discuss in greater detail how structs are declared later.
+
 ### Names in Mojom
 
-Names in Mojom are not important. Except in affecting compatibility at level of
-source code (when generating bindings), names in a Mojom file may be changed
+Names in Mojom are not important. Except in affecting compatibility at the level
+of source code (when generating bindings), names in a Mojom file may be changed
 arbitrarily without any effect on the "meaning" of the Mojom file (subject to
 basic language requirements, e.g., avoiding collisions with keywords and other
 names). E.g., the following is completely equivalent to the interface discussed
 above:
 ```mojom
 interface Something {
   One(int32 an_integer, string a_string);
   Two() => (bool a_boolean, uint32 an_unsigned);
   Three() => ();
 };
@@ skipping 21 matching lines @@
 };
 ```
 
 Ordinals also tie into the versioning scheme (**TODO(vtl)**: link?), which
 allows Mojom files to be evolved in a backwards-compatible way. We will not
 discuss this matter further here.
 
 **TODO(vtl)**: Maybe mention exceptions to this in attributes (e.g.,
 `ServiceName`).
 
+## Mojom files
+
+A Mojom file consists of, in order:
+* an optional *module* declaration;
+* zero or more *import* statements (the order of these is not important); and
+* zero or more declarations of *struct*s, *interface*s, *union*s, *enum*s, or
+  *const*s (the order of these is not important).
+(These are all described further below.)
+
+Additionally, C/C++-style comments are supported (i.e., single-line comments
+starting with `//` or multi-line comments of the form `/* ... */`).
+
+As stated above, the order of struct/interface/union/enum/const declarations is
+not important. This is required to allow "cyclic" structures to be defined.
+Nonetheless, whenever possible, one should declare things before they are
+"used". For example, the following is valid but not recommended:
+```mojom
+// NOT recommended.
+
+const MyEnum kMyConst = kMyOtherConst;
+const MyEnum kMyOtherConst = A_VALUE;
+
+enum MyEnum {
+  A_VALUE,
+  ANOTHER_VALUE,
+};
+```
+
+### Naming style
+
+There is a standard style for naming things:
+* `StudlyCaps` (a.k.a. `CapitalizedCamelCase`) for: (struct, interface, union,
+  and enum) type names and message (a.k.a. function or method) names;
+* `unix_hacker_style` for field names (in structs and unions) and "parameter"
+  names;
+* `ALL_CAPS_UNIX_HACKER_STYLE` for enum value names; and
+* `kStudlyCaps` for const names.
+
+Following this style is highly recommended, since code generators for various
+languages will expect this style, in order to transform the names into a more
+language-appropriate style.
+
+### Module statement
+
+**TODO(vtl)**
+
+### Import declarations
+
+**TODO(vtl)**
+
+### Struct declarations
+
+A Mojom struct declaration consists of a finite sequence of *field declaration*,
+each of which consists of a *type*, a *name*, and optionally a *default value*
+(if applicable for the given type). (If no default value is declared, then the
+default is the default value for the field type, typically 0, null, or similar.)
+
+Additionally, a struct may contain enum and const declarations (**TODO(vtl)**:
+why not struct/union/interface declarations?). While the order of the field
+declarations (with respect to one another) is important, the ordering of the
+enum/const declarations (with respect to both the field declarations and other
+enum/const declarations) is not. (But as before, we recommend declaring things
+before "use".)
+
+Here is an example with these elements:
+```mojom
+struct Foo {
+  const int8 kSomeConstant = 123;
+
+  enum MyEnum {
+    A_VALUE,
+    ANOTHER_VALUE
+  };
+
+  int8 first_field = kSomeConstant;
+  uint32 second_field = 123;
+  MyEnum etc_etc = A_VALUE;
+  float a;    // Default value is 0.
+  string? b;  // Default value is null.
+};
+```
+(Note that `kSomeConstant` may be referred to as `Foo.kSomeConstant` and,
+similarly, `MyEnum` as `Foo.MyEnum`. This is required outside of the `Foo`
+declaration.)
+
+### Interface declarations
+
+**TODO(vtl)**
+
+### Union declarations
+
+**TODO(vtl)**
+
+### Enum declarations
+
+**TODO(vtl)**
+
+### Const declarations
+
+**TODO(vtl)**
 
 **TODO(vtl)**: Write/(re)organize the sections below.
 
-## Structs
-
-## Modules
-
 ## Data types
 
 ### Primitive types
 
-### Enums
+#### Standard types
 
-### Strings
+#### Enum types
 
-### Nullability
+### "Pointer" types
 
-### Structs
+#### Nullability
 
-### Arrays
+#### Strings
 
-### Maps
+#### Maps
 
-### Unions
+#### Structs
 
-### Handle values
+#### Arrays
 
-### Interface values
+#### Unions
 
-### Interface requests
+### Handle types
 
-## Consts
+#### Raw handle types
+
+#### Interface types
+
+#### Interface request types
 
 ## Annotations
 
 ## Pipelining