relnotes: Add mention of QuantifiedConstraints

docs/users_guide/8.6.1-notes.rst

@@ -28,6 +28,19 @@ Full details
 
 Language
 ~~~~~~~~
+
+- Use of quantified type variables in constraints is now allowed via the
+  :extension:`QuantifiedConstraints` language extension. This long-awaited feature
+  enables users to encode significantly more precision in their types. For instance,
+  the common ``MonadTrans`` typeclass could now make the expectation that an
+  applied transformer is must be a ``Monad`` ::
+
+      class (forall a. Monad m => Monad (t m)) => MonadTrans t where {- ... -}
+
+  Additionally, quantification can enable terminating instance resolution
+  where this previously was not possible. See :ref:`quantified-constraints` for
+  details.
+
 - A new :extension:`DerivingVia` language extension has been added which allows
   the use of the ``via`` deriving strategy. For instance: ::
 

docs/users_guide/glasgow_exts.rst

@@ -9707,9 +9707,18 @@ contexts and superclasses, but to do so you must use
 :extension:`UndecidableInstances` to signal that you don't mind if the type
 checker fails to terminate.
 
+.. _quantified-constraints:
+
 Quantified constraints
 ======================
 
+.. extension:: QuantifiedConstraints
+    :shortdesc: Allow ``forall`` quantifiers in constraints.
+
+    :since: 8.6.1
+
+    Allow constraints to quantify over types.
+
 The extension :extension:`QuantifiedConstraints` introduces **quantified constraints**,
 which give a new level of expressiveness in constraints. For example, consider ::