net/http: document use of DetectContentType

Fixes #2365.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5653070

src/pkg/net/http/server.go

@@ -59,7 +59,9 @@ type ResponseWriter interface {
 
 	// Write writes the data to the connection as part of an HTTP reply.
 	// If WriteHeader has not yet been called, Write calls WriteHeader(http.StatusOK)
-	// before writing the data.
+	// before writing the data.  If the Header does not contain a
+	// Content-Type line, Write adds a Content-Type set to the result of passing
+	// the initial 512 bytes of written data to DetectContentType.
 	Write([]byte) (int, error)
 
 	// WriteHeader sends an HTTP response header with status code.

src/pkg/net/http/sniff.go

@@ -9,15 +9,15 @@ import (
 	"encoding/binary"
 )
 
-// Content-type sniffing algorithm.
+// The algorithm uses at most sniffLen bytes to make its decision.
-// References in this file refer to this draft specification:
-//   http://mimesniff.spec.whatwg.org/
-
-// The algorithm prefers to use sniffLen bytes to make its decision.
 const sniffLen = 512
 
-// DetectContentType returns the sniffed Content-Type string
+// DetectContentType implements the algorithm described
-// for the given data. This function always returns a valid MIME type.
+// at http://mimesniff.spec.whatwg.org/ to determine the
+// Content-Type of the given data.  It considers at most the
+// first 512 bytes of data.  DetectContentType always returns
+// a valid MIME type: if it cannot determine a more specific one, it
+// returns "application/octet-stream".
 func DetectContentType(data []byte) string {
 	if len(data) > sniffLen {
 		data = data[:sniffLen]