Document how tokio::sync::broadcast::channel(0) behaves

[TheButlah]

I'm not sure how broadcast::channel(0) would work, and if I should set my capacity to 0 or 1 if I only intend on ever sending 1 message. If the capacity is 0, does that mean the sending thread blocks until all receiving threads have received, like crossbeam_channel? https://docs.rs/crossbeam-channel/latest/crossbeam_channel/struct.Sender.html#method.send

[Darksonn]

The function will panic if you specify a capacity of zero.

Feel free to submit a PR that adds that to the documentation.