Update v2 endpoint show help

https://review.openstack.org/#/c/319821/ updated v2 endpoint show
and the doc, but not the v3 help.

Change-Id: Ifaa90f6266eabafb9de544199759f4ee8bcc1c83

openstackclient/identity/v3/endpoint.py

@@ -243,8 +243,9 @@ class ShowEndpoint(command.ShowOne):
         parser = super(ShowEndpoint, self).get_parser(prog_name)
         parser.add_argument(
             'endpoint',
-            metavar='<endpoint-id>',
+            metavar='<endpoint>',
-            help='Endpoint ID to display',
+            help='Endpoint to display (endpoint ID, service ID,'
+                 ' service name, service type)',
         )
         return parser