doc: add explanation why keep var with for loop

This comment will help contributors to understand why keeping var
PR-URL: #30380
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

lib/internal/async_hooks.js

@@ -129,6 +129,8 @@ function emitInitNative(asyncId, type, triggerAsyncId, resource) {
   active_hooks.call_depth += 1;
   // Use a single try/catch for all hooks to avoid setting up one per iteration.
   try {
+    // Using var here instead of let because "for (var ...)" is faster than let.
+    // Refs: https://github.com/nodejs/node/pull/30380#issuecomment-552948364
     for (var i = 0; i < active_hooks.array.length; i++) {
       if (typeof active_hooks.array[i][init_symbol] === 'function') {
         active_hooks.array[i][init_symbol](
@@ -159,6 +161,8 @@ function emitHook(symbol, asyncId) {
   // Use a single try/catch for all hook to avoid setting up one per
   // iteration.
   try {
+    // Using var here instead of let because "for (var ...)" is faster than let.
+    // Refs: https://github.com/nodejs/node/pull/30380#issuecomment-552948364
     for (var i = 0; i < active_hooks.array.length; i++) {
       if (typeof active_hooks.array[i][symbol] === 'function') {
         active_hooks.array[i][symbol](asyncId);