[1.9.x] Fixed #26831 -- Documented session data must be JSON encodable for JSONSerializer.

Backport of 1f82b857ceb75f2d7a68e79c6a00c30bfe7f1318 from master

docs/topics/http/sessions.txt

@@ -354,6 +354,9 @@ Bundled serializers
         >>> request.session['0']
         'bar'
 
+    Similarly, data that can't be encoded in JSON, such as non-UTF8 bytes like
+    ``'\xd9'`` (which raises :exc:`UnicodeDecodeError`), can't be stored.
+
     See the :ref:`custom-serializers` section for more details on limitations
     of JSON serialization.