std: Update documentation on the `comm` module

Some of this documentation got a little out of date. There was no mention of a
`SyncSender`, and the entire "Outside the runtime" section isn't really true any
more (or really all that relevant).

This also updates a few other doc blocks and adds some examples.
auto merge of #13465 : alexcrichton/rust/fix-comm-dox, r=brson

Some of this documentation got a little out of date. There was no mention of a
`SyncSender`, and the entire "Outside the runtime" section isn't really true any
more (or really all that relevant).

This also updates a few other doc blocks and adds some examples.