Document the new style instance group filters

Document the ServerGroupAffinityFilter and ServerGroupAntiAffinityFilter
filters.

Add a note about the deprecated GroupAffinityFilter and
GroupAntiAffinityFilter filters. Also indicate that those filters can't
work together.

Update some markup.

Add notes about the role of the scheduler during migration and
evacuation.

Change-Id: Ibc7989baa336eed698704ca2de38e04f027269b0
Closes-Bug: #1298980
Closes-Bug: #1299066
Merge "Document the new style instance group filters"