net: Add examples to UnixDatagram (#2765)

* net: adding examples for UnixDatagram

Adding examples to documentation for UnixDatagram

* net: document named UnixDatagrams persistence

Add documentation to indicate that named UnixDatagrams 'leak'
socket files after execution.

* net: rustfmt issue in UnixDatagram

Fixing rustfmt issue in UnixDatagram

* net: adding examples for UnixDatagram

Fixes: #2686
Refs: #1679
Refs: #1111