recv_deadline with past instant is ambiguous

[AurelienFT]

Hello,

Thanks for your work on this crate.
I find recv_deadline on a channel a bit ambiguous when we pass an Instant that is in the past. This behavior isn't documented and, in my first intuition was: it will not even try to recv as the deadline is already pass. After a test it shows me that even if the deadline is in the past but there is already a message in the channel he will be received.

Do you think the behavior should change or is it possible to document it ?

[taiki-e]

"deadline" relates to whether or not to wait for a message for how long, and I think it is reasonable to receive it if the message is immediately available.

I think it is reasonable to document the behavior.