* 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
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