DOCS: add note on opposite not being possible
DOCS: add note on opposite not being possible

mnicholl/MOSFiT,bmockler/MOSFiT,bmockler/MOSFiT,villrv/MOSFiT,guillochon/FriendlyFit,guillochon/MOSFiT,villrv/MOSFiT,mnicholl/MOSFiT,guillochon/MOSFiT,guillochon/MOSFiT,mnicholl/MOSFiT