Expand/clarify documentation of stm_become_inevitable.

Created originally on Bitbucket by Devin Jeanpierre

Was already merged in Bitbucket before import, marked as merged by the import user

Sorry for the anemic PR. I'm still learning the way stmgc works, made my first working program using it and noticed this along the way.

Main thing I wanted to fix was that the doc comment was not actually (directly) above the function it was documenting. Feel free to rewrite as necessary.