Rules updated (CAN/MAY, MUST/CANNOT)

[JC Lawrence]

Changelog:

- Many small formatting and wording tweaks.
- Rationalised the use of CAN vs MAY, including adding a few necessary MUSTs and CANNOTs.
- Rewrote and clarified the section on transferring presidencies to other players.

I'd normally not post such a small change, but there's a related discussion around use of CAN vs MAY in rules on BGG that references these rules.

-- JCL