Document that mpmc channels deliver an item to one receiver.

See discussion:

rust-lang#126840 (comment)

Author: andrew-otiv

library/std/src/sync/mpmc/mod.rs

@@ -9,6 +9,7 @@
 //! [`Sender`]s are used to send data to a set of [`Receiver`]s. Both
 //! sender and receiver are cloneable (multi-producer) such that many threads can send
 //! simultaneously to receivers (multi-consumer).
+//! Each item is delivered to (at most) one receiver.
 //!
 //! These channels come in two flavors:
 //!