document onTilesLoaded

[jaketangosierra]

I stumbled across this event, which does exactly what I was looking for, by reading the source code (because it felt like it should exist, it seemed like a logical event to have). It would be useful if it were documented somewhere.

I can make a pull request for this if necessary, but I figured I'd open a ticket.

[DanielShinken]

thank you for this, had to read code source too, but it works and it is extremely useful!