fixes #505

As mentioned in my previous pull request, this is an extension of the work done there to make it so when we unset a group badge, it keeps the same "registered" date, which matters because of the price bump.
This is indeed a docstring, which means it is available at runtime, but only resides in memory in one place, rather than being copied with every object that gets instantiated or anything like that.

In this case I felt like the comment was necessary because a newcomer who saw that there was a ``delete_from_group`` function might legitimately wonder why they couldn't just say ``session.delete(attendee)`` like they do for any other type of object.  So a comment seemed necessary.

I don't plan to add docstrings for every single method, just anything complicated or counterintuitive.