doc: added warning for ordering of dynamic classes

This PR addresses this issue: https://github.com/kivy/kivy/issues/1791

I initially addressed this with a PR to prevent it. https://github.com/kivy/kivy/pull/1813

This broke the showcase and other examples. On further thought, the name "dynamic classes" seems to imply very flexible, on-the-fly class composition. This ability makes dynamic classes really flexible, so why not consider it a feature? Added documentation accordingly.