add domain scope arguments to v3 role add in doc

There are optional domain scope arguments --user-domain,
--group-domain and --project-domain to filter user, group
and project for command 'os role add', however, the doc
is missing them.

Closes-Bug: #1460296
Change-Id: Ie7c7707d183da042c51e98b6cd4003c89efc4032

Author: Guojian Shao

doc/source/command-objects/role.rst

@@ -13,8 +13,8 @@ Add role to a user or group in a project or domain
 .. code:: bash
 
     os role add
-        --domain <domain> | --project <project>
-        --user <user> | --group <group>
+        --domain <domain> | --project <project> [--project-domain <project-domain>]
+        --user <user> [--user-domain <user-domain>] | --group <group> [--group-domain <group-domain>]
         <role>
 
 .. option:: --domain <domain>