net/http: document Error does not end the request

Fixes #15205

Change-Id: Ia650806756758ca8ed2272b1696e59b809b16c61
Reviewed-on: https://go-review.googlesource.com/21836Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/server.go

@@ -1652,6 +1652,8 @@ func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
 // Helper handlers
 
 // Error replies to the request with the specified error message and HTTP code.
+// It does not otherwise end the request; the caller should ensure no further
+// writes are done to w.
 // The error message should be plain text.
 func Error(w ResponseWriter, error string, code int) {
 	w.Header().Set("Content-Type", "text/plain; charset=utf-8")