net: better documentation of UNIX sockets

[alandonovan]

The documentation on how to use the various net.Unix* features is rather thin. I don't mind reading man pages for the underlying system calls, but it's often unclear what the abstractions actually do in terms of those system calls. Some substantial examples would help, including ones that show how to pass a file descriptor from one process to another, and which combinations of read/write functions are appopriate for stream, datagram, and packet-oriented sockets.