test: refactor comments in test-child-process-spawnsync-maxbuf

* remove comment that isn't relevant/important
* add comment that explains what the test does

PR-URL: https://github.com/nodejs/node/pull/16829
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-child-process-spawnsync-maxbuf.js

@@ -1,10 +1,12 @@
 'use strict';
 require('../common');
+
+// This test checks that the maxBuffer option for child_process.spawnSync()
+// works as expected.
+
 const assert = require('assert');
 const spawnSync = require('child_process').spawnSync;
 const msgOut = 'this is stdout';
-
-// This is actually not os.EOL?
 const msgOutBuf = Buffer.from(`${msgOut}\n`);
 
 const args = [