net: update documentation on Dial and its variants

net: update documentation on Dial and its variants

This change clarifies the documentation on Dial and its variants 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.
Fixes #17614.
Fixes #17738.
Fixes #17956.
Updates #18806.

Change-Id: I6adb3f2ae04a3bf83b96016ed73d8e59926f3e8a
Reviewed-on: https://go-review.googlesource.com/34875
Reviewed-by: Ian Lance Taylor <iant@golang.org>