net: update documentation on Listen and ListenPacket

net: update documentation on Listen and ListenPacket

This change clarifies the documentation on Listen and ListenPacket to
avoid unnecessary confusion about how the arguments for the connection
setup functions are used to make connections.

Also replaces "name" or "hostname" with "host name" when the term
implies the use of DNS.

Updates #17613.
Updates #17614.
Updates #17615.
Fixes #17616.
Updates #17738.
Updates #17956.

Change-Id: I0bad2e143207666f2358d397fc076548ee6c3ae9
Reviewed-on: https://go-review.googlesource.com/34876
Reviewed-by: Ian Lance Taylor <iant@golang.org>