Include supported migration scenarios in the ipa-to-ipa docs
    
    Lay out the supported migration paths. Users are likely to get
    creative with migration so we need to narrow the path for which
    we can provide support.
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>