Rollup merge of rust-lang#143447 - jieyouxu:compiletest-maintenance-4, r=Kobzol

[COMPILETEST-UNTANGLE 4/N] Improve compiletest config documentation

This is part of a patch series to untangle `compiletest` to hopefully nudge it towards being more maintainable.

This PR should contain **no functional changes**.

This is pulled out to its own PR to make follow-up changes easier to review.

There are *intentionally* a *lot* of FIXME comments, intended to be gradually addressed in follow-ups.

r? `@Kobzol`

Author: saethlin