net/http: add more docs on ErrHijacked

Updates #16456

Change-Id: Ifea651ea3ece2267a6f0c1638181d6ddd9248a9f
Reviewed-on: https://go-review.googlesource.com/31181Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/server.go

@@ -40,7 +40,9 @@ var (
 
 	// ErrHijacked is returned by ResponseWriter.Write calls when
 	// the underlying connection has been hijacked using the
-	// Hijacker interfaced.
+	// Hijacker interface. A zero-byte write on a hijacked
+	// connection will return ErrHijacked without any other side
+	// effects.
 	ErrHijacked = errors.New("http: connection has been hijacked")
 
 	// ErrContentLength is returned by ResponseWriter.Write calls