Auto merge of #52936 - felixrabe:patch-1, r=alexcrichton

Auto merge of #52936 - felixrabe:patch-1, r=alexcrichton

Document #39364 – Panic in mpsc::Receiver::recv_timeout

I can still reproduce #39364 with the example code at https://github.com/rust-lang/rust/issues/39364#issuecomment-320637702.

I'm opening this PR in an attempt to document this bug as a known issue in [libstd/sync/mpsc/mod.rs](https://github.com/rust-lang/rust/blob/master/src/libstd/sync/mpsc/mod.rs).

Inputs very much welcome. ([Nightly docs for `recv_timeout`.](https://doc.rust-lang.org/nightly/std/sync/mpsc/struct.Receiver.html?search=#method.recv_timeout))