Add additional comment for --extra-gn-args in build_aar.py.

Bug: webrtc:9003
Change-Id: I6387b097b13b82477bd161093c00985070147953
Reviewed-on: https://webrtc-review.googlesource.com/61323
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22402}
diff --git a/tools_webrtc/android/build_aar.py b/tools_webrtc/android/build_aar.py
index 0ddca2f..5a7b56e 100755
--- a/tools_webrtc/android/build_aar.py
+++ b/tools_webrtc/android/build_aar.py
@@ -66,7 +66,9 @@
   parser.add_argument('--verbose', action='store_true', default=False,
       help='Debug logging.')
   parser.add_argument('--extra-gn-args', default=[], nargs='*',
-      help='Additional GN args to be used during Ninja generation.')
+      help="""Additional GN arguments to be used during Ninja generation.
+              These are applied after any other arguments and will
+              override any values defined by the script.""")
   return parser.parse_args()