net/http: be clear that HTTPS is supported

Fixes #6443.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/14231045

src/pkg/net/http/doc.go

@@ -5,7 +5,7 @@
 /*
 Package http provides HTTP client and server implementations.
 
-Get, Head, Post, and PostForm make HTTP requests:
+Get, Head, Post, and PostForm make HTTP (or HTTPS) requests:
 
 	resp, err := http.Get("http://example.com/")
 	...