Merge pull request #12641 from ichard26/docs/legacy

Clearly document the legacy resolver as unsupported

Author: sbidoul

docs/html/user_guide.rst

@@ -1140,6 +1140,13 @@ Since this work will not change user-visible behavior described in the
 pip documentation, this change is not covered by the :ref:`Deprecation
 Policy`.
 
+.. attention::
+
+    The legacy resolver is deprecated and unsupported. New features, such
+    as :doc:`reference/installation-report`, will not work with the
+    legacy resolver and this resolver will be removed in a future
+    release.
+
 Context and followup
 --------------------