net: fix comment on IPv4bcast

In Go 1.8, almost all the platforms except NaCl provide network
interface and address identification and applications can use IPv4
limited or directed broadcast addresses appropriately.

Fixes #18176.

Change-Id: Ie5de834d19c0aaeb4128a3ca655f6c4c9ae5e501
Reviewed-on: https://go-review.googlesource.com/34435Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/ip.go

@@ -90,7 +90,7 @@ func CIDRMask(ones, bits int) IPMask {
 
 // Well-known IPv4 addresses
 var (
-	IPv4bcast     = IPv4(255, 255, 255, 255) // broadcast
+	IPv4bcast     = IPv4(255, 255, 255, 255) // limited broadcast
 	IPv4allsys    = IPv4(224, 0, 0, 1)       // all systems
 	IPv4allrouter = IPv4(224, 0, 0, 2)       // all routers
 	IPv4zero      = IPv4(0, 0, 0, 0)         // all zeros