Improve Control.Monad.guard and Control.Monad.MonadPlus docs

This fixes Issue #12372: documentation for Control.Monad.guard not
useful after AMP.

Reviewers: hvr, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4258

(cherry picked from commit 6847c6bf)