Placement client: clean up docstrings
Placement client: clean up docstrings

Many :raises: and :returns: are missing from the docstrings.
This patch adds them and clarifies the wording in a few places.

Change-Id: If642cf94b04f63bfaca1eb2f6c8c3c6e27c26c00
Related-Bug: #1578989

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib