net: BUG section for ReadFrom, ReadFromIP methods of IPConn on "ip4" network

Fixes issue 3944.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13263043
net: make protocol-specific Dial and Listen return consistent error value

Update issue 4856

R=golang-dev, bradfitz, dave
CC=golang-dev
https://codereview.appspot.com/12916046