feat: add summary for understanding processnexttick doc

Author: kekekuli

apps/site/pages/en/learn/asynchronous-work/understanding-processnexttick.md

@@ -52,4 +52,6 @@ Running at order 3 or 4, setImmediate
 Running at order 3 or 4, setTimeout
 ```
 
-The exact output may differ from run to run. [setImmediate vs setTimeout](https://nodejs.org/en/learn/asynchronous-work/event-loop-timers-and-nexttick#setimmediate-vs-settimeout) explain the reason.
+In that case, `console.log('Hello => number 1');` will first run because event loop only starts after call stack is cleared.The `nextTick` queue is processed before entering the next phase, which is why the `process.nextTick()` callback runs immediately after.
+
+The execution order of `serImmediate` and `setTimeout` can not be determined in the main module. For a detailed explanation, refer to [setImmediate vs setTimeout](https://nodejs.org/en/learn/asynchronous-work/event-loop-timers-and-nexttick#setimmediate-vs-settimeout).