net: sync CIDRMask code, doc

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4961069

src/pkg/net/ip.go

@@ -65,8 +65,9 @@ func IPv4Mask(a, b, c, d byte) IPMask {
 	return p
 }
 
-// CIDRMask returns an IPMask of size bits with the first n bits
-// set to to 1. nil is returned if the mask is impossible.
+// CIDRMask returns an IPMask consisting of `ones' 1 bits
+// followed by 0s up to a total length of `bits' bits.
+// For a mask of this form, CIDRMask is the inverse of IPMask.Size.
 func CIDRMask(ones, bits int) IPMask {
 	if bits != 8*IPv4len && bits != 8*IPv6len {
 		return nil