Add support for customizing GN's args text.

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 1aa56f1d4542e1623bb07afb7f12549834472f46..749148071c7c1e4ac849d8b98961a98941a3355b 100644
--- a/tools/gn/docs/reference.md
+++ b/tools/gn/docs/reference.md
@@ -5846,6 +5846,10 @@
 
       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**