MonadComprehensions docs should not claim 'guard' requires MonadPlus

Summary

Location of documentation issue: the GHC user's guide: https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/monad_comprehensions.html

The docs claim that 'guard' needs MonadPlus, but it only needs Alternative. 'guard' was changed in base 4.8 released in Apr 2015 to use the 'new' Alternative class (which was added in base 4.4, GHC 7.2.1 from Aug 2011).

Proposed improvements or changes

It should reference 'Alternative' instead of MonadPlus.

Environment

master

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