Change --size helptext to include backup

Followup from [1]. Modifying help text of --size
argument to include --backup option.

[1] https://review.opendev.org/c/openstack/python-openstackclient/+/864893

Change-Id: I12cf60079ebcfe1cd059602fbfc1a13c8fe86803

Author: rajathere

openstackclient/volume/v2/volume.py

@@ -93,8 +93,8 @@ def get_parser(self, prog_name):
             "--size",
             metavar="<size>",
             type=int,
-            help=_("Volume size in GB (Required unless --snapshot or "
-                   "--source is specified)"),
+            help=_("Volume size in GB (required unless --snapshot, "
+                   "--source or --backup is specified)"),
         )
         parser.add_argument(
             "--type",