Add documentation for GroupAntiAffinityFilter

fixes bug 1128587

With Grizzly comes the GroupAntiAffinityFilter, which ensures that
instances in a group don't end up on the same hosts.

This patch explains the filter and its scheduler hint

Change-Id: I9395f07d8fdbfa9d6c705a4769c775c8e9f15ad5
Merge "Add documentation for GroupAntiAffinityFilter"