Fixes for r37489:

Fixes for r37489:
* Don't pass $data or $flags by reference.  They're never accessed again, so it makes no sense.
* Document in hooks.txt.
The interface for this hook seems very messy.  It seems like it would be much cleaner to expose $data as $url (null if stored locally); and $flags as an array, or something like that.  If the text is gzipped, the text can be extracted from the Revision and re-gzipped if you really want to for some strange reason.