which is `cabal repl --with-ghc=doctest`. This not only catches up with
the current upstream recommendations, but also simplifies the usage:
before this change calling doctest required you to set up an environment
with QuickCheck available (we used to do it via cabal-env). The new
method takes care of it automatically.

Also add some docs to fix #8147

(cherry picked from commit 822d1a73)