Clarify how watch::Sender subscribe() and closed() methods interact

[geeklint]

Version
Observed in docs for 1.37.0.

Description

The cancel safety section of the documentation for watch::Sender::closed reads

This method is cancel safe. Once the channel is closed, it stays closed forever and all future calls to closed will return immediately.

In the time since that documentation was written, watch::Sender::subscribe has been added, which allows re-opening the channel. That sentence should presumably be removed, and it would also be nice if the docs clarified how closed() interacts with subscribe(), particularly when called concurrently on different threads (is my assumption correct that the two methods can race and by the time closed() returns, a new Receiver may already exist?)