Add notes regarding flat=True

Start documenting that flat=True and aliased=True don't work
with selectable particularly when selectable is an aliased select
already.  References #4212

Change-Id: I6e576165f06387350ae97e43ad979e575a4912b9
(cherry picked from commit 39d7dfa08a)

lib/sqlalchemy/orm/util.py

@@ -698,7 +698,10 @@ def with_polymorphic(base, classes, selectable=False,
         This can be important when using the with_polymorphic()
         to create the target of a JOIN on a backend that does not
         support parenthesized joins, such as SQLite and older
-        versions of MySQL.
+        versions of MySQL.   However if the
+        :paramref:`.with_polymorphic.selectable` parameter is in use
+        with an existing :class:`.Alias` construct, then you should not
+        set this flag.
 
     :param flat: Boolean, will be passed through to the
      :meth:`.FromClause.alias` call so that aliases of :class:`.Join`