Clarify command-like help for check --only.

borgmatic/commands/arguments.py

@@ -218,7 +218,7 @@ def parse_arguments(*unparsed_arguments):
         choices=('repository', 'archives', 'data', 'extract'),
         dest='only',
         action='append',
-        help='Run a particular consistency check instead of configured checks (can specify multiple times)',
+        help='Run a particular consistency check (repository, archives, data, or extract) instead of configured checks; can specify flag multiple times',
     )
     check_group.add_argument('-h', '--help', action='help', help='Show this help message and exit')