Clarify how many bits mcontrol/mcontrol6 compare. (#604)

* Clarify how many bits mcontrol/mcontrol6 compare.

Addresses #569.

* Address the easy feedback.

* Clarify that trigger values are always 0-extended.

* Hopefully make things less ambiguous.

Which is, unfortunately, not the same as making them more clear.

* Minor rephrase.