Warn that "switch back" option breaks AJAX forms with custom callbacks

Many modules, such as Panels and Media, use custom AJAX callbacks for various purposes. If the option to "switch back to http pages when there are no matches" is enabled, and you haven't added all of these callbacks to the list of secure pages, they will die silently and without explanation.

For instance:

1. Enable the Media browser for a file or content type, and configure admin pages to use SSL and switch to HTTP for non-admin pages.
2. Go to add a node or file.
3. Click the 'select' button to launch the media browser. You'll simply get a blank white modal, with no error messages. This is because securepages is trying to force the callback to use http.

I think a bit of documentation is a good fix to start- just a simple warning below the option to switch back.