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
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 ```
 ## **depfile**: [string] File name for input dependencies for actions.
 
 ```
   If nonempty, this string specifies that the current action or
   action_foreach target will generate the given ".d" file containing
   the dependencies of the input. Empty or unset means that the script
   doesn't generate the files.
 
+  A depfile should be used only when a target depends on files that are
+  not already specified by a target's inputs and sources. Likewise,
+  depfiles should specify only those dependencies not already included
+  in sources or inputs.
+
   The .d file should go in the target output directory. If you have more
   than one source file that the script is being run over, you can use
   the output file expansions described in "gn help action_foreach" to
   name the .d file according to the input.
-  The format is that of a Makefile, and all of the paths should be
-  relative to the root build directory.
+  The format is that of a Makefile and all paths must be relative to the
+  root build directory. Only one output may be listed and it must match
+  the first output of the action.
+
+  Although depfiles are created by an action, they should not be listed
+  in the action's "outputs" unless another target will use the file as
+  an input.
 
 ```
 
 ### **Example**
 
 ```
   action_foreach("myscript_target") {
     script = "myscript.py"
     sources = [ ... ]
 
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       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}}
+      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.
+        "//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.
 
 ```