Document what fast render mode does

[rcombs]

This was added recently but it's not clear how it actually works, and what kind of loss can be expected when using it.

[LukePulverenti]

Thanks for updating the notes. What you've written suggests that provided no bitmaps fail, rendering will be equivalent. Is that correct?

[TFSThiagoBR98]

Yes, for example, it would be as a player, playing a video along with a heavy process, it will start to drop frames to keep playing without locking.