Harts can be in exactly 1 of 4 states. (#354)

* Harts can be in exactly 1 of 4 states.

Fixes #351.

* Grammar fixes.

* While held in reset, harts are unavailable.

Fixes #353.

* Remove "impl specific [reset] indicators"

They don't supersede the unavailable status, and their mention is
confusing. (They were added in ce1b3593.)

Fixes #353.

* Unavailable harts can't execute abstract commands.

Being in reset doesn't mean you have to be unavailable. #355

* Clarify reset states.

* Abstract behavior when hart is/becomes unavailable

Just a rewording that (hopefully) makes more sense.