net: update documentation for FileConn, FilePacketConn, FileListener
net: update documentation for FileConn, FilePacketConn, FileListener

Closes the API documentation gap between platforms.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8086043
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go