Support for source_target_relative expansion in GN

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 6565 matching lines @@
       source file is in a different directory than the BUILD.gn file.
         "//foo/bar/baz.txt" => "gen/foo/bar"
 
   {{source_out_dir}}
       The object file directory (*) corresponding to the source file's path,
       relative to the build directory. this us be different than the target's
       out directory if the source file is in a different directory than the
       build.gn file.
         "//foo/bar/baz.txt" => "obj/foo/bar"
 
+  {{source_target_relative}}\n"
+      The path to the source file relative to the target's directory. This will
+      generally be used for replicating the source directory layout in the
+      output directory. This can only be used in actions and it is an error to
+      use in process_file_template where there is no "target".
+        "//foo/bar/baz.txt" => "baz.txt"
+
 ```
 
 ### **(*) Note on directories**
 
 ```
   Paths containing directories (except the source_root_relative_dir) will be
   different depending on what context the expansion is evaluated in. Generally
   it should "just work" but it means you can't concatenate strings containing
   these values with reasonable results.
 
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 **  \--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.
 
 ```