watch: document recursive `borrow` deadlock (#4360)
watch: document recursive `borrow` deadlock (#4360)

Under the hood, the watch channel uses a RwLock to implement reading
(borrow) and writing (send). This may cause a deadlock if a user has
concurrent borrows on the same thread. This is most likely to occur  due
to a recursive borrow.

This PR adds documentation to describe the deadlock so that future users
of the watch channel will be aware.
tokio-rs/tokio,tokio-rs/tokio