user-guide: Mention COMPLETE pragma in release notes

Reviewers: austin

Subscribers: rwbarton, thomie

GHC Trac Issues: #14305

Differential Revision: https://phabricator.haskell.org/D4059

docs/users_guide/8.2.1-notes.rst

@@ -54,6 +54,10 @@ Language
 - Support for overloaded record fields via a new ``HasField`` class and
   associated compiler logic (see :ref:`record-field-selector-polymorphism`)
 
+- GHC now recognizes the ``COMPLETE`` language pragma, allowing the user to
+  specify sets of patterns (including pattern synonyms) which constitute a
+  complete pattern match. See :ref:`complete-pragma` for details.
+
 Compiler
 ~~~~~~~~