Since `-XPatternGuards` is enabled by default, invert the logic and
mention `-XNoPatternGuards` first.

Also, since this is a Haskell 2010 feature, refer to the language report
instead of explaining it.

In general, I would like to follow the guideline of assuming the latest
language report as a given, and then only report GHC's deviations and
extensions relative to that report.

Reviewed by: bgamari

Differential Revision: https://phabricator.haskell.org/D2319

GHC Trac Issues: #12172