Refactoring RbacUtils part 3 - documentation

Change the documnentation according to new RbacUtilsMixin

Story: 2002604
Task: 22223

Change-Id: I30ab8ea002f9312a5b50e2f2c511ed321a679c00
diff --git a/doc/source/multi-policy-validation.rst b/doc/source/multi-policy-validation.rst
index 576fd68..441bd35 100644
--- a/doc/source/multi-policy-validation.rst
+++ b/doc/source/multi-policy-validation.rst
@@ -66,7 +66,7 @@
       self.os_admin.servers_client.lock_server(self.server['id'])
       self.addCleanup(self.servers_client.unlock_server, self.server['id'])
 
-      with self.rbac_utils.override_role(self):
+      with self.override_role():
           self.servers_client.unlock_server(self.server['id'])
 
 While the ``expected_error_codes`` parameter is omitted in the example above,
@@ -96,7 +96,7 @@
         # Verify specific fields of a port
         fields = ['binding:vif_type']
 
-        with self.rbac_utils.override_role(self):
+        with self.override_role():
             retrieved_port = self.ports_client.show_port(
                 self.port['id'], fields=fields)['port']
 
@@ -131,7 +131,7 @@
 
         RBAC test for the neutron create_network:router:external policy
         """
-        with self.rbac_utils.override_role(self):
+        with self.override_role():
             self._create_network(router_external=True)
 
 Note that above the following expected error codes/rules relationship is
@@ -158,7 +158,7 @@
 
         RBAC test for the neutron update_network:shared policy
         """
-        with self.rbac_utils.override_role(self):
+        with self.override_role():
             self._update_network(shared_network=True)
         self.addCleanup(self._update_network, shared_network=False)