Add help the ability to sort images with multiple keys

Related Change: If79779a4c52c8dc5c4f39192d3d247335a76ba24
This help is also used for OpenStack Command-Line Interface Reference.

Change-Id: Iadce779afebe4aa80026e46f169546aba9055477
Partial-Bug: #1432813

glanceclient/v2/shell.py

@@ -138,7 +138,8 @@ def do_image_update(gc, args):
            help=_("Filter images by a user-defined tag."))
 @utils.arg('--sort-key', default=[], action='append',
            choices=images.SORT_KEY_VALUES,
-           help=_('Sort image list by specified fields.'))
+           help=_('Sort image list by specified fields.'
+                  ' May be used multiple times.'))
 @utils.arg('--sort-dir', default=[], action='append',
            choices=images.SORT_DIR_VALUES,
            help=_('Sort image list in specified directions.'))