filter_mathjaxloader needs to explain why it works the way it does

filter_mathjaxloader works in a particular way, but one that is not at all obvious (to me at least).

I assume damyon did it that way for a very good reason, but unfortunately the reasons why it is what way only seems to be recorded in Damyon's brain. It really should be recorded in the PHPdocs, or inline comments, in that code, so that when other people try to maintain that code, they know what they are doing.

Thanks.