Improve docs for UdpSocket::set_nonblocking.
Improve docs for UdpSocket::set_nonblocking.

Closes https://github.com/rust-lang/rust/issues/44050.

aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust