Revise content on lang feature processes

Let's revise both the new content in this PR as well as the remaining
text in the relevant chapters so as to better describe our current
processes related to language features and their stabilizations.

For the new stabilization report template, based on reviewing its
early use and well as reviewing earlier stabilization reports, we've
revised it editorially and added questions designed to capture
additional details to which we commonly want people to speak.

Author: traviscross