net: update documentation on IP.IsUnspecified

Fixes #19344.

Change-Id: Ic6fc7485cb50bfae99fda69d0cd9c4ae434af4c3
Reviewed-on: https://go-review.googlesource.com/44910Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/ip.go

@@ -108,7 +108,8 @@ var (
 	IPv6linklocalallrouters    = IP{0xff, 0x02, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x02}
 )
 
-// IsUnspecified reports whether ip is an unspecified address.
+// IsUnspecified reports whether ip is an unspecified address, either
+// the IPv4 address "0.0.0.0" or the IPv6 address "::".
 func (ip IP) IsUnspecified() bool {
 	return ip.Equal(IPv4zero) || ip.Equal(IPv6unspecified)
 }