Fix sinks and toilets not draining (#33691)

Fix sinks and toilets not draining (#33691)

* Fix AutoDrain

Per the system comments, AutoDrain is designed to automatically move
puddles into the drain (like a floor drain). Drains without AutoDrain
are still supposed to gradually empty the buffer, but not remove puddles
(like sinks and toilets).

However, a logic error in the original implementation causes drains with
AutoDrain set to false to simply not work. Hence sinks never emptied.

* Update documentation