users-guide: Clarify GHC2021 documentation

Point out that GHC2021 doesn't offer the same degree of stability that
Haskell2010 does, as noted by @phadej.

docs/users_guide/exts/control.rst

@@ -18,7 +18,7 @@ Language extensions can be controlled (i.e. allowed or not) in two ways:
    ``{-# LANGUAGE TemplateHaskell #-}`` (see :ref:`language-pragma`).
 
 .. extension:: GHC2021
-    :shortdesc: Use GHC’s set of default languages from 2021
+    :shortdesc: Use GHC’s set of default language extensions from 2021
 
     GHC blesses a number of extensions, beyond Haskell 2010, to be suitable to
     turned on by default. These extensions are considered to be stable and
@@ -29,6 +29,12 @@ Language extensions can be controlled (i.e. allowed or not) in two ways:
     ``GHC20xx`` by default, users are advised to declare the language set
     explicitly with ``-XGHC2021``.
 
+    Note that, because GHC2021 includes a number of non-standardized
+    extensions, the stability guarantees it provides are not quite as strong as
+    those provided by, e.g., :extension:`Haskell2010`. While GHC does take
+    pains to avoid changing the semantics of these extensions, changes may
+    still happen (e.g. the simplified subsumption change introduced in GHC 9.0
+    which caused GHC to reject some programs using :extension:`RankNTypes`).
 
     The ``GHC2021`` language set comprises the following extensions: