net: update documentation for ListenTCP, ListenUDP

Adds the missing wildcard port assignment description to ListenUDP.
Also updates the wildcard port description on ListenTCP.

R=golang-dev, dave, r
CC=golang-dev
https://codereview.appspot.com/8063043
net: close TCPListener in example

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/8073044
api: remove Zone of net.IPNet from next.txt

Update issue 4234.
Update issue 4501.

R=golang-dev, dave, bradfitz
CC=golang-dev
https://codereview.appspot.com/8129043