Clearly document the legacy resolver as unsupported

Author: ichard26

docs/html/user_guide.rst

@@ -1140,6 +1140,12 @@ 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`, may not function correctly
+    with the legacy resolver. **These issues will not be fixed.**
+
 Context and followup
 --------------------