net/http: document ResponseWriter.WriteHeader more

Change-Id: I65209b90ed7c56d4c751b3e4b3ce1de52dae368c
Reviewed-on: https://go-review.googlesource.com/82635Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/server.go

@@ -132,12 +132,20 @@ type ResponseWriter interface {
 	// possible to maximize compatibility.
 	Write([]byte) (int, error)
 
-	// WriteHeader sends an HTTP response header with status code.
+	// WriteHeader sends an HTTP response header with the provided
+	// status code.
+	//
 	// If WriteHeader is not called explicitly, the first call to Write
 	// will trigger an implicit WriteHeader(http.StatusOK).
 	// Thus explicit calls to WriteHeader are mainly used to
 	// send error codes.
-	WriteHeader(int)
+	//
+	// The provided code must be a valid HTTP 1xx-5xx status code.
+	// Only one header may be written. Go does not currently
+	// support sending user-defined 1xx informational headers,
+	// with the exception of 100-continue response header that the
+	// Server sends automatically when the Request.Body is read.
+	WriteHeader(statusCode int)
 }
 
 // The Flusher interface is implemented by ResponseWriters that allow