net: improve IPMask docs

Fixes #28957

Change-Id: Ie8ba841bd4ee71766bcfbbfbdc9173b9be867ed1
Reviewed-on: https://go-review.googlesource.com/c/go/+/151479Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/ip.go

@@ -31,7 +31,10 @@ const (
 // be an IPv4 address.
 type IP []byte
 
-// An IP mask is an IP address.
+// An IPMask is a bitmask that can be used to manipulate
+// IP addresses for IP addressing and routing.
+//
+// See type IPNet and func ParseCIDR for details.
 type IPMask []byte
 
 // An IPNet represents an IP network.