Update os_image documentation (#49705)

* Update os_image.py docs to show auth details

lib/ansible/modules/cloud/openstack/os_image.py

@@ -91,6 +91,8 @@ EXAMPLES = '''
       username: admin
       password: passme
       project_name: admin
+      os_user_domain_name: Default
+      os_project_domain_name: Default
     name: cirros
     container_format: bare
     disk_format: qcow2

lib/ansible/plugins/doc_fragments/openstack.py

@@ -24,7 +24,8 @@ options:
       - Dictionary containing auth information as needed by the cloud's auth
         plugin strategy. For the default I(password) plugin, this would contain
         I(auth_url), I(username), I(password), I(project_name) and any
-        information about domains if the cloud supports them. For other plugins,
+        information about domains (for example, I(os_user_domain_name) or I(os_project_domain_name)) if the cloud supports them.
+        For other plugins,
         this param will need to contain whatever parameters that auth plugin
         requires. This parameter is not needed if a named cloud is provided or
         OpenStack OS_* environment variables are present.