gh-77024: test.support: Improve documentation (GH-92513)

This is a rework of GH-5774 on current main. I was a bit more
conservative in making changes than the original PR.

See @csabella's comments on issue GH-77024 and the discussion
on GH-5774 for explanations of several of the changes.

Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>
Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
(cherry picked from commit 89951770)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>