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

docs/users_guide/glasgow_exts.rst

@@ -12523,6 +12523,10 @@ optionally had by adding ``!`` in front of a variable.
    Adding ``~`` in front of ``x`` gives the regular lazy behavior.
+   Turning patterns into irrefutable ones requires ``~(~p)`` or ``(~ ~p)`` when ``Strict`` is enabled.
+
+   
+
 -  **Let/where bindings**
    When the user writes ::