Mention #15050 in the release notes for 8.8.1

docs/users_guide/8.8.1-notes.rst

@@ -22,6 +22,10 @@ Full details
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+- :extension:`ScopedTypeVariables`: The type variable that a type signature on
+  a pattern can bring into scope can now stand for arbitrary types. Previously,
+  they could only stand in for other type variables, but this restriction was deemed
+  unnecessary in `GHC proposal #29 <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0029-scoped-type-variables-types.rst>`__. Also see :ghc-ticket:`15050`.
 
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