net: update documentation on Dial{TCP,UDP,IP,Unix}

net: update documentation on Dial{TCP,UDP,IP,Unix}

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

Change-Id: I2e378182948fbe221f6ae786ab55e77ae90c3f3b
Reviewed-on: https://go-review.googlesource.com/34877
Reviewed-by: Ian Lance Taylor <iant@golang.org>