Rollup merge of #41438 - projektir:mpsc_docs, r=steveklabnik
Rollup merge of #41438 - projektir:mpsc_docs, r=steveklabnik

Adding links and examples for various mspc pages #29377

Adding links and copying examples for the various Iterators; adding some extra stuff to `Sender`/`SyncSender`/`Receiver`.

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