Document filters for get methods
Document filters for get methods

Add documentation for the filter parameter.

Change-Id: I862760181b353906dbda8785eb05cde216cc17c0

openstack-infra/shade,stackforge/python-openstacksdk,openstack/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,dtroyer/python-openstacksdk,openstack-infra/shade,dtroyer/python-openstacksdk