net: use IPv4/IPv6 reserved address blocks for documentation

Also replaces google.com with golang.org in package documentation.

Updates #15228.

Change-Id: I554fa960878fa44557a522635ed412d8d7548d3f
Reviewed-on: https://go-review.googlesource.com/23126
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/net/net.go b/src/net/net.go
index 27e9ca3..d6812d1 100644
--- a/src/net/net.go
+++ b/src/net/net.go
@@ -14,7 +14,7 @@
 
 The Dial function connects to a server:
 
-	conn, err := net.Dial("tcp", "google.com:80")
+	conn, err := net.Dial("tcp", "golang.org:80")
 	if err != nil {
 		// handle error
 	}