More descriptive git cl split --help info about $directory

R=iannucci@chromium.org

Bug:
Change-Id: Icf44fd71f1521a9e7f0eb8c4710924a5e45b541b
Reviewed-on: https://chromium-review.googlesource.com/759131
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>

git_cl.py

@@ -4979,7 +4979,8 @@ def CMDsplit(parser, args):
   the shared OWNERS file.
   """
   parser.add_option("-d", "--description", dest="description_file",
-                    help="A text file containing a CL description. ")
+                    help="A text file containing a CL description in which "
+                         "$directory will be replaced by each CL's directory.")
   parser.add_option("-c", "--comment", dest="comment_file",
                     help="A text file containing a CL comment.")
   options, _ = parser.parse_args(args)