Document why we notify all streams when sending a single message

[Ten0]

async-channel/src/lib.rs

Lines 204 to 208 in a0ba218

	// it will notify another blocked receive operation.
	self.channel.recv_ops.notify_additional(1);
	// Notify all blocked streams.
	self.channel.stream_ops.notify(usize::MAX);

It's unclear to me why all streams are woken up instead of a single one when sending a message. (That is, why don't streams and recv behave in the same way, when the stream is typically essentially "polling recv forever").

Would it be possible to document that further?

Thanks,