Current documentation of `sync::watch::Sender::send` may be intepreted
as the method failing if at some point in past the channel has been
closed because every receiver has been dropped. This isn't true,
however, as the channel could have been reopened by using
`sync::watch::Sender::subscribe`.

This fix clarifies the behavior. Moreover, it is noted that on failure,
the value isn't made available to future subscribers (but returned as
part of the `SendError`).

Fixes tokio-rs#4957.

sync: doc of watch::Sender::send improved

Current documentation of `sync::watch::Sender::send` may be intepreted
as the method failing if at some point in past the channel has been
closed because every receiver has been dropped. This isn't true,
however, as the channel could have been reopened by using
`sync::watch::Sender::subscribe`.

This fix clarifies the behavior. Moreover, it is noted that on failure,
the value isn't made available to future subscribers (but returned as
part of the `SendError`), and that the other send methods
(`send_if_modified`, `send_modify`, or `send_replace`) can be used to
always make a new value available for future receivers, even if no
receiver currently exists.

Fixes tokio-rs#4957.