Modify the help message for 'registered limit set'

Regsitered limit set CLI takes --service, --region and
--resource-name as param which can be updated along with
--default limit for existing registered limit.

Default limit can be updated with same value and CLI
return the success. But --service, --region and --resource-
name cannot be same as existing one. CLI return 409
for this case. Which is valid behaviour because more than
one limit with same service and same resource cannot exist.

But help message of --service, --region and --resource-name
are not much clear to tell that they cannot be passed with
same value.

This patch clarifies the help message for resigtered limit set CLI.

Reference Scenario:
*  openstack registered limit set --default-limit 91 64c2e97fbe904b888544ffdcab21989b
limit updated sucessfully

Updating limit with exsiting service and resource-name:

* openstack registered limit set --default-limit 92 --resource-name snapshot 64c2e97fbe904b888544ffdcab21989b
Conflict occurred attempting to store registered_limit - Duplicate entry. (HTTP 409)

*openstack registered limit set --default-limit 93 --service compute 64c2e97fbe904b888544ffdcab21989b
Conflict occurred attempting to store registered_limit - Duplicate entry. (HTTP 409)

*openstack registered limit set --default-limit 91 --resource-name snapshot --service glance 64c2e97fbe904b888544ffdcab21989b
Conflict occurred attempting to store registered_limit - Duplicate entry. (HTTP 409)

Change-Id: I9e78a6250567cd981adde96946818bb016760a49

Author: vishakha-codes

openstackclient/identity/v3/registered_limit.py

@@ -186,12 +186,18 @@ def get_parser(self, prog_name):
         parser.add_argument(
             '--service',
             metavar='<service>',
-            help=_('Service responsible for the resource to limit'),
+            help=_('Service to be updated responsible for the resource to '
+                   'limit. Either --service, --resource-name or --region must '
+                   'be different than existing value otherwise it will be '
+                   'duplicate entry')
         )
         parser.add_argument(
             '--resource-name',
             metavar='<resource-name>',
-            help=_('The name of the resource to limit'),
+            help=_('Resource to be updated responsible for the resource to '
+                   'limit. Either --service, --resource-name or --region must '
+                   'be different than existing value otherwise it will be '
+                   'duplicate entry'),
         )
         parser.add_argument(
             '--default-limit',
@@ -202,12 +208,15 @@ def get_parser(self, prog_name):
         parser.add_argument(
             '--description',
             metavar='<description>',
-            help=_('Description of the registered limit'),
+            help=_('Description to update of the registered limit'),
         )
         parser.add_argument(
             '--region',
             metavar='<region>',
-            help=_('Region for the registered limit to affect.'),
+            help=_('Region for the registered limit to affect. Either '
+                   '--service, --resource-name or --region must be '
+                   'different than existing value otherwise it will be '
+                   'duplicate entry'),
         )
         return parser