Add a //nogncheck anntation to GN include checker

tools/gn/docs/reference.md

Index: tools/gn/docs/reference.md
diff --git a/tools/gn/docs/reference.md b/tools/gn/docs/reference.md
index c55f42e05b394327fe6bb25e2d7d6a0715f91954..ad3ac85676af7484335a90a07b9eeb3dbfb8eeb0 100644
--- a/tools/gn/docs/reference.md
+++ b/tools/gn/docs/reference.md
@@ -1011,6 +1011,7 @@
 ```
   Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
          defines, include_dirs, ldflags, lib_dirs, libs
+         precompiled_header, precompiled_source
 
 ```
 
@@ -1209,6 +1210,7 @@
 ```
   Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
          defines, include_dirs, ldflags, lib_dirs, libs
+         precompiled_header, precompiled_source
   Deps: data_deps, deps, forward_dependent_configs_from, public_deps
   Dependent configs: all_dependent_configs, public_configs
   General: check_includes, configs, data, inputs, output_name,
@@ -1920,6 +1922,7 @@
 ```
   Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
          defines, include_dirs, ldflags, lib_dirs, libs
+         precompiled_header, precompiled_source
   Deps: data_deps, deps, forward_dependent_configs_from, public_deps
   Dependent configs: all_dependent_configs, public_configs
   General: check_includes, configs, data, inputs, output_name,
@@ -1960,6 +1963,7 @@
 ```
   Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
          defines, include_dirs, ldflags, lib_dirs, libs
+         precompiled_header, precompiled_source
   Deps: data_deps, deps, forward_dependent_configs_from, public_deps
   Dependent configs: all_dependent_configs, public_configs
   General: check_includes, configs, data, inputs, output_name,
@@ -1983,6 +1987,7 @@
 ```
   Flags: cflags, cflags_c, cflags_cc, cflags_objc, cflags_objcc,
          defines, include_dirs, ldflags, lib_dirs, libs
+         precompiled_header, precompiled_source
   Deps: data_deps, deps, forward_dependent_configs_from, public_deps
   Dependent configs: all_dependent_configs, public_configs
   General: check_includes, configs, data, inputs, output_name,
@@ -2292,6 +2297,20 @@
         Posix systems:
           output_prefix = "lib"
 
+    precompiled_header_type  [string]
+        Valid for: "cc", "cxx", "objc", "objcxx"
+
+        Type of precompiled headers. If undefined or the empty string,
+        precompiled headers will not be used for this tool. Otherwise
+        use "msvc" which is the only currently supported value.
+
+        For precompiled headers to be used for a given target, the
+        target (or a config applied to it) must also specify a
+        "precompiled_header" and, for "msvc"-style headers, a
+        "precompiled_source" value.
+
+        See "gn help precompiled_header" for more.
+
     restat  [boolean]
         Valid for: all tools (optional, defaults to false)
 
@@ -2332,6 +2351,7 @@
 ```
 
 ### **Expansions for tool variables**
+
 ```
   All paths are relative to the root build directory, which is the
   current directory for running all tools. These expansions are
@@ -3242,6 +3262,30 @@
 
 ```
 
+### **Controlling includes individually**
+
+```
+  If only certain includes are problematic, you can annotate them
+  individually rather than disabling header checking on an entire
+  target. Add the string "nogncheck" to the include line:
+
+    #include "foo/something_weird.h"  // nogncheck (bug 12345)
+
+  It is good form to include a reference to a bug (if the include is
+  improper, or some other comment expressing why the header checker
+  doesn't work for this particular case.
+
+  The most common reason to need "nogncheck" is conditional includes.
+  The header checker does not understand the preprocessor, so may flag
+  some includes as improper even if the dependencies and #defines are
+  always matched correctly:
+
+    #if defined(ENABLE_DOOM_MELON)
+    #include "doom_melon/beam_controller.h"  // nogncheck
+    #endif
+
+```
+
 ### **Example**
 
 ```
@@ -3772,6 +3816,77 @@
 
 
 ```
+## **precompiled_header**: [string] Header file to precompile.
+
+```
+  Precompiled headers will be used when a target specifies this
+  value, or a config applying to this target specifies this value.
+  In addition, the tool corresponding to the source files must also
+  specify precompiled headers (see "gn help tool"). The tool
+  will also specify what type of precompiled headers to use.
+
+  The precompiled header/source variables can be specified on a target
+  or a config, but must be the same for all configs applying to a given
+  target since a target can only have one precompiled header.
+
+```
+
+### **MSVC precompiled headers**
+
+```
+  When using MSVC-style precompiled headers, the "precompiled_header"
+  value is a string corresponding to the header. This is NOT a path
+  to a file that GN recognises, but rather the exact string that appears
+  in quotes after an #include line in source code. The compiler will
+  match this string against includes or forced includes (/FI).
+
+  MSVC also requires a source file to compile the header with. This must
+  be specified by the "precompiled_source" value. In contrast to the
+  header value, this IS a GN-style file name, and tells GN which source
+  file to compile to make the .pch file used for subsequent compiles.
+
+  If you use both C and C++ sources, the precompiled header and source
+  file will be compiled using both tools. You will want to make sure
+  to wrap C++ includes in __cplusplus #ifdefs so the file will compile
+  in C mode.
+
+  For example, if the toolchain specifies MSVC headers:
+
+    toolchain("vc_x64") {
+      ...
+      tool("cxx") {
+        precompiled_header_type = "msvc"
+        ...
+
+  You might make a config like this:
+
+    config("use_precompiled_headers") {
+      precompiled_header = "build/precompile.h"
+      precompiled_source = "//build/precompile.cc"
+
+      # Either your source files should #include "build/precompile.h"
+      # first, or you can do this to force-include the header.
+      cflags = [ "/FI$precompiled_header" ]
+    }
+
+  And then define a target that uses the config:
+
+    executable("doom_melon") {
+      configs += [ ":use_precompiled_headers" ]
+      ...
+
+
+
+```
+## **precompiled_source**: [file name] Source file to precompile.
+
+```
+  The source file that goes along with the precompiled_header when
+  using "msvc"-style precompiled headers. It will be implicitly added
+  to the sources of the target. See "gn help precompiled_header".
+
+
+```
 ## **public**: Declare public header files for a target.
 
 ```