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

Backport of 1f82b857ce from master

docs/topics/http/sessions.txt

@@ -348,6 +348,9 @@ 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.