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

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

See @csabella's comments on issue #77024 and the discussion
on #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>