docs: update description for AggregateInstanceExtraSpecsFilter
docs: update description for AggregateInstanceExtraSpecsFilter

This copies a line from the ComputeCapabilitiesFilter description
which points out that if the flavor does not have any extra specs
then it passes the filter. I had to figure this out via code inspection
so it would be good to document it.

Change-Id: Id7b3ec5dd2847c9ea615981031d7e0f2b1beb3e2

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