Cleanup docstring in test_l3_agent_scheduler

The docstring was incomplete in saying it required the
l3_agent_scheduler extension to be enabled.  I think it probably meant
to say that l3_agent_scheduler was listed in
network_feature_enabled.api_extensions, but that defaults to 'all' so
rather than explain how that config option works in the docstring, just
say that the l3_agent_scheduler extension is required and assume the
user understands how to configure Tempest.

Change-Id: If504bd6374b83dea24fa92f9ef6c852d76308ad1
diff --git a/tempest/api/network/admin/test_l3_agent_scheduler.py b/tempest/api/network/admin/test_l3_agent_scheduler.py
index f4050c5..3b05f42 100644
--- a/tempest/api/network/admin/test_l3_agent_scheduler.py
+++ b/tempest/api/network/admin/test_l3_agent_scheduler.py
@@ -28,8 +28,9 @@
         List L3 agents hosting the given router.
         Add and Remove Router to L3 agent
 
-    v2.0 of the Neutron API is assumed. It is also assumed that the following
-    options are defined in the [network] section of etc/tempest.conf:
+    v2.0 of the Neutron API is assumed.
+
+    The l3_agent_scheduler extension is required for these tests.
     """
 
     @classmethod