net: update documentation for IPConn and related stuff

Closes the API documentation gap between platforms.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8143044
net: update documentation for TCPConn, TCPListener and related stuff

Closes the API documentation gap between platforms.
Also makes the code textual representation same between platforms.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8147043
net: update documentation for UDPConn and related stuff

Closes the API documentation gap between platforms.
Also makes the code textual representation same between platforms.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8148043
net: update documentation for UnixConn, UnixListener and related stuff

Closes the API documentation gap between platforms.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8063044