net: update documentation on Listen{TCP,UDP,MulticastUDP,IP,Unix,Unixgram}

net: update documentation on Listen{TCP,UDP,MulticastUDP,IP,Unix,Unixgram}

This change clarifies the documentation on
Listen{TCP,UDP,MulticastUDP,IP,Unix,Unixgram} to avoid unnecessary
confusion about how the arguments for the connection setup functions
are used to make connections.

Change-Id: Ie269453ef49ec2db893391dc3ed2f7b641c14249
Reviewed-on: https://go-review.googlesource.com/34878
Reviewed-by: Ian Lance Taylor <iant@golang.org>