This change replaces the remaining use of "mask" with "prefix length"
and uses IPv4 reserved address blocks for documentation.

UPdates #15228.
Updates #18175.

Change-Id: I56c4f1205821c64f3195b023ad515b9d54e33f64
Reviewed-on: https://go-review.googlesource.com/34431
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>