gh-59956: Clarify Runtime State Status Expectations (gh-101308)

A PyThreadState can be in one of many states in its lifecycle, represented by some status value.  Those statuses haven't been particularly clear, so we're addressing that here.  Specifically:

* made the distinct lifecycle statuses clear on PyThreadState
* identified expectations of how various lifecycle-related functions relate to status
* noted the various places where those expectations don't match the actual behavior

At some point we'll need to address the mismatches.

(This change also includes some cleanup.)

https://github.com/python/cpython/issues/59956