Clarify that watch channels are closed for non-persistent watches

[ilikeorangutans]

Methods setting watches return channels with events. If the watches are non-persistent the channels will be closed after the watch fires. It is important to stop reading from the channel after the event because once it's closed, reading from the channel will produce events in their default state which is confusing.

The documentation of the watch methods should warn that the channels close and further reads will produce confusing events.

CC @jpfourny