Support for source_target_relative expansion in GN

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 ab3233aa4ed5f32d6e2e2bf2388477a6fc5cbea8..39054c8073117dd8bf389039e04fdde8bf458d89 100644
--- a/tools/gn/docs/reference.md
+++ b/tools/gn/docs/reference.md
@@ -6583,6 +6583,13 @@
       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**