net: fix ParseCIDR docs

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>

src/net/ip.go

@@ -658,9 +658,10 @@ func ParseIP(s string) IP {
 // like "192.0.2.0/24" or "2001:db8::/32", as defined in
 // RFC 4632 and RFC 4291.
 //
-// It returns the IP address and the network implied by the IP
-// and mask. For example, ParseCIDR("198.51.100.1/24") returns
-// the IP address 198.51.100.1 and the network 198.51.100.0/24.
+// It returns the IP address and the network implied by the IP and
+// prefix length.
+// For example, ParseCIDR("192.0.2.1/24") returns the IP address
+// 198.0.2.1 and the network 198.0.2.0/24.
 func ParseCIDR(s string) (IP, *IPNet, error) {
 	i := byteIndex(s, '/')
 	if i < 0 {