http: add comment about outputSize in res/server

PR-URL: https://github.com/nodejs/node/pull/3128

lib/_http_server.js

@@ -265,6 +265,10 @@ function connectionListener(socket) {
   var outgoingData = 0;
 
   function updateOutgoingData(delta) {
+    // `outgoingData` is an approximate amount of bytes queued through all
+    // inactive responses. If more data than the high watermark is queued - we
+    // need to pause TCP socket/HTTP parser, and wait until the data will be
+    // sent to the client.
     outgoingData += delta;
     if (socket._paused && outgoingData < socket._writableState.highWaterMark)
       return socketOnDrain();