api-guide: better explain scheduler hints.
api-guide: better explain scheduler hints.

This patch explains what scheduler hints are, gives an example
request, then links to the api-ref for details.

Co-Authored-By: Artom Lifshitz <alifshit@redhat.com>

Change-Id: I1c55b9a17e9d9f57d9ec1383680cccb6ae7fcfaf

openstack/nova,openstack/nova,mahak/nova,openstack/nova,rahulunair/nova,mahak/nova,rahulunair/nova,klmitch/nova,mahak/nova,klmitch/nova,rahulunair/nova,klmitch/nova,klmitch/nova