Add filter explain in api ref about parents_as_list and subtree_as_list
Add filter explain in api ref about parents_as_list and subtree_as_list

I create a project and its child, then show the project with subtree_as_list,
but the subtree list in response is null.
I can not find problem via api ref about this parameter, until reading code about
subtree_as_list. The subtree list will be filtered against the projects this
user has access to. So the filter explain should add to the api ref.

Change-Id: If5c1c8dff05c81be180d5d4858fadf9ce7051ef8

mahak/keystone,openstack/keystone,rajalokan/keystone,openstack/keystone,rajalokan/keystone,rajalokan/keystone,openstack/keystone,mahak/keystone,mahak/keystone