Fix sphinx warnings from signature_utils
Fix sphinx warnings from signature_utils

Spotted some warnings that don't appear to fail the build being output
for the doc string in signature utils. This change fixes that.

TrivialFix

Change-Id: Ied6908019fe9640deebcb78bdd2ed77cd2fb7101

klmitch/nova,bigswitch/nova,CEG-FYP-OpenStack/scheduler,mahak/nova,cernops/nova,mahak/nova,klmitch/nova,gooddata/openstack-nova,openstack/nova,dims/nova,BeyondTheClouds/nova,rahulunair/nova,rahulunair/nova,dims/nova,rajalokan/nova,vmturbo/nova,rahulunair/nova,mahak/nova,phenoxim/nova,cloudbase/nova,klmitch/nova,jianghuaw/nova,NeCTAR-RC/nova,sebrandon1/nova,cernops/nova,NeCTAR-RC/nova,mikalstill/nova,mikalstill/nova,zhimin711/nova,jianghuaw/nova,openstack/nova,cyx1231st/nova,sebrandon1/nova,hanlind/nova,Juniper/nova,rajalokan/nova,gooddata/openstack-nova,alaski/nova,openstack/nova,gooddata/openstack-nova,rajalokan/nova,mikalstill/nova,jianghuaw/nova,rajalokan/nova,BeyondTheClouds/nova,cernops/nova,BeyondTheClouds/nova,bigswitch/nova,alaski/nova,jianghuaw/nova,Juniper/nova,Juniper/nova,vmturbo/nova,vmturbo/nova,Juniper/nova,cloudbase/nova,phenoxim/nova,gooddata/openstack-nova,cloudbase/nova,klmitch/nova,hanlind/nova,hanlind/nova,sebrandon1/nova,vmturbo/nova,CEG-FYP-OpenStack/scheduler,cyx1231st/nova,zhimin711/nova