Provide documentation for title option

Perhaps this is a linux convention but it was not obvious to me that
'y' would use the default.

Change-Id: I5aedcb365828015effa64d41153f6cb9c9318dfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5580992
Commit-Queue: Peter McNeeley <petermcneeley@chromium.org>
Reviewed-by: Yiwei Zhang <yiwzhang@google.com>

git_cl.py

@@ -1852,8 +1852,8 @@ class Changelist(object):
         title = RunGit(['show', '-s', '--format=%s', 'HEAD', '--']).strip()
         if options.force or options.skip_title:
             return title
-        user_title = gclient_utils.AskForData('Title for patchset [%s]: ' %
-                                              title)
+        user_title = gclient_utils.AskForData(
+            'Title for patchset (\'y\' for default) [%s]: ' % title)
 
         # Use the default title if the user confirms the default with a 'y'.
         if user_title.lower() == 'y':