Documentation: document the mandatory field behavior

Make it clear how mandatory field absence should be affect the policy.

Change-Id: Iba2f96ec6edca93002dd79ca7c66c4f1ab411141
Bug-Url: https://bugzilla.redhat.com/1143992
Signed-off-by: Roy Golan <rgolan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/35011
Reviewed-by: Martin Sivák <msivak@redhat.com>
Reviewed-by: Adam Litke <alitke@redhat.com>
Tested-by: Adam Litke <alitke@redhat.com>