api-ref: Further clarify placement aggregates
api-ref: Further clarify placement aggregates

Following up on the comments at [1], provide further disambiguation and
clarity on the difference between placement aggregates and nova host
aggregates, with some illustrative examples that include where the
former may be used to represent the latter (as is being proposed via
[2]).

[1] https://review.openstack.org/#/c/539033/1/placement-api-ref/source/aggregates.inc@14
[2] https://review.openstack.org/#/q/bp/placement-mirror-host-aggregates

Change-Id: I53588fca311992f885c4157591232a864b6a86b1

openstack/nova,klmitch/nova,gooddata/openstack-nova,mikalstill/nova,gooddata/openstack-nova,rahulunair/nova,klmitch/nova,gooddata/openstack-nova,phenoxim/nova,mahak/nova,openstack/nova,mahak/nova,gooddata/openstack-nova,openstack/nova,mikalstill/nova,mikalstill/nova,mahak/nova,rahulunair/nova,rahulunair/nova,klmitch/nova,klmitch/nova,phenoxim/nova