lib: adding perf notes js_stream_socket.js

Leaving var in place of let for performance optimization in short loops in hot paths. Added comments explaining why.

lib/internal/js_stream_socket.js

@@ -157,6 +157,7 @@ class JSStreamSocket extends Socket {
     let pending = bufs.length;
 
     this.stream.cork();
+    // Use `var` over `let` for performance optimization.
     for (var i = 0; i < bufs.length; ++i)
       this.stream.write(bufs[i], done);
     this.stream.uncork();