Update OS-EP-FILTER API

Update the API doc for the Keystone extension OS-EP-FILTER with the
new Endpoint Grouping APIs.

Related spec: https://review.openstack.org/#/c/102023/

Change-Id: If69f9d5c74ebf185f62974ad2d1da26778c6f15b
Merge "Update OS-EP-FILTER API"