Update GN documentation.

tools/gn/docs/reference.md

 # GN Reference
 
 *This page is automatically generated from* `gn help --markdown all`.
 
 ## **\--args**: Specifies build arguments overrides.
 
 ```
   See "gn help buildargs" for an overview of how build arguments work.
 
   Most operations take a build directory. The build arguments are taken
@@ skipping 664 matching lines @@
   XML file contains a list of include paths and defines. Because GN does
   not generate a full .cproject definition, it is not possible to
   properly define includes/defines for each file individually.
   Instead, one set of includes/defines is generated for the entire
   project. This works fairly well but may still result in a few indexer
   issues here and there.
 
 
 ```
 ## **gn help <anything>**
+
 ```
   Yo dawg, I heard you like help on your help so I put help on the help
   in the help.
 
+  You can also use "all" as the parameter to get all help at once.
+
+```
+
+### **Switches**
+
+```
+  --markdown
+      Format output in markdown syntax.
+
+```
+
+### **Example**
+
+```
+  gn help --markdown all
+      Dump all help to stdout in markdown format.
+
 
 ```
 ## **gn ls <out_dir> [<label_pattern>] [\--all-toolchains] [\--as=...]**
 ```
       [--type=...] [--testonly=...]
 
   Lists all targets matching the given pattern for the given build
   directory. By default, only targets in the default toolchain will
   be matched unless a toolchain is explicitly supplied.
 
@@ skipping 886 matching lines @@
 
 ```
 ## **foreach**: Iterate over a list.
 
 ```
   foreach(<loop_var>, <list>) {
     <loop contents>
   }
 
   Executes the loop contents block over each item in the list,
-  assigning the loop_var to each item in sequence.
+  assigning the loop_var to each item in sequence. The loop_var will be
+  a copy so assigning to it will not mutate the list.
 
   The block does not introduce a new scope, so that variable assignments
   inside the loop will be visible once the loop terminates.
 
   The loop variable will temporarily shadow any existing variables with
   the same name for the duration of the loop. After the loop terminates
   the loop variable will no longer be in scope, and the previous value
   (if any) will be restored.
 
 ```
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 **  -q**: Quiet mode. Don't print output on success.
 **  \--root**: Explicitly specify source root.
 **  \--runtime-deps-list-file**: Save runtime dependencies for targets in file.
 **  \--threads**: Specify number of worker threads.
 **  \--time**: Outputs a summary of how long everything took.
 **  \--tracelog**: Writes a Chrome-compatible trace log to the given file.
 **  -v**: Verbose logging.
 **  \--version**: Prints the GN version number and exits.
 
 ```