http: add docs/warning on incorrect use of NewChunkedWriter

R=golang-dev, adg, r
CC=golang-dev
https://golang.org/cl/4536075

src/pkg/http/chunked.go

@@ -6,6 +6,7 @@ package http
 
 import (
 	"io"
+	"log"
 	"os"
 	"strconv"
 )
@@ -13,7 +14,16 @@ import (
 // NewChunkedWriter returns a new writer that translates writes into HTTP
 // "chunked" format before writing them to w. Closing the returned writer
 // sends the final 0-length chunk that marks the end of the stream.
+//
+// NewChunkedWriter is not needed by normal applications. The http
+// package adds chunking automatically if handlers don't set a
+// Content-Length header. Using NewChunkedWriter inside a handler
+// would result in double chunking or chunking with a Content-Length
+// length, both of which are wrong.
 func NewChunkedWriter(w io.Writer) io.WriteCloser {
+	if _, bad := w.(*response); bad {
+		log.Printf("warning: using NewChunkedWriter in an http.Handler; expect corrupt output")
+	}
 	return &chunkedWriter{w}
 }