Add support for customizing GN's args text.

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 from
@@ skipping 5828 matching lines @@
       for a BUILD.gn file (or the build config file discussed above), the file
       will first be looked for in the source root. If it's not found, the
       secondary source root will be checked (which would contain a parallel
       directory hierarchy).
 
       This behavior is intended to be used when BUILD.gn files can't be checked
       in to certain source directories for whatever reason.
 
       The secondary source root must be inside the main source tree.
 
+  arg_file_template [optional]
+      Label of a file that contains the text that should be used as the
+      default args.gn file when you run `gn args`.
+
 ```
 
 ### **Example .gn file contents**
 
 ```
   buildconfig = "//build/config/BUILDCONFIG.gn"
 
   check_targets = [
     "//doom_melon/*",  # Check everything in this subtree.
     "//tools:mind_controlling_ant",  # Check this specific target.
@@ skipping 786 matching lines @@
 **  \--root**: Explicitly specify source root.
 **  \--runtime-deps-list-file**: Save runtime dependencies for targets in file.
 **  \--script-executable**: Set the executable used to execute scripts.
 **  \--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.
 
 ```