Add information about irrefutable pattern Syntax to XStrict.
Add information about irrefutable pattern Syntax to XStrict.

This information was present in the ghc wiki but not the user guide.
[skip ci]

Test Plan: None

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: RyanGlScott, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4250

shlevy/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc