Update docstring of _schema_validation_helper
Update docstring of _schema_validation_helper

During review of this new helper method there were some comments
about clarifying the docstring for usage. This patch adds the
clarification.

Change-Id: I15177a55768423ae57dce244070de026c34c9418

Juniper/nova,mahak/nova,gooddata/openstack-nova,Juniper/nova,rajalokan/nova,mikalstill/nova,rahulunair/nova,phenoxim/nova,mikalstill/nova,rajalokan/nova,jianghuaw/nova,klmitch/nova,phenoxim/nova,klmitch/nova,rahulunair/nova,hanlind/nova,rahulunair/nova,mahak/nova,vmturbo/nova,gooddata/openstack-nova,rajalokan/nova,gooddata/openstack-nova,mahak/nova,openstack/nova,Juniper/nova,hanlind/nova,hanlind/nova,vmturbo/nova,jianghuaw/nova,vmturbo/nova,openstack/nova,jianghuaw/nova,Juniper/nova,gooddata/openstack-nova,vmturbo/nova,klmitch/nova,mikalstill/nova,jianghuaw/nova,klmitch/nova,openstack/nova,rajalokan/nova