test: add comment describing test-fs-mkdir

PR-URL: #22424
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>

test/parallel/test-fs-mkdir.js

@@ -33,6 +33,7 @@ function nextdir() {
   return `test${++dirc}`;
 }
 
+// mkdir creates directory using assigned path
 {
   const pathname = path.join(tmpdir.path, nextdir());
 
@@ -42,6 +43,7 @@ function nextdir() {
   }));
 }
 
+// mkdir creates directory with assigned mode value
 {
   const pathname = path.join(tmpdir.path, nextdir());
 
@@ -51,6 +53,7 @@ function nextdir() {
   }));
 }
 
+// mkdirSync successfully creates directory from given path
 {
   const pathname = path.join(tmpdir.path, nextdir());
 
@@ -60,6 +63,8 @@ function nextdir() {
   assert.strictEqual(exists, true);
 }
 
+// mkdirSync and mkdir require path to be a string, buffer or url.
+// Anything else generates an error.
 [false, 1, {}, [], null, undefined].forEach((i) => {
   common.expectsError(
     () => fs.mkdir(i, common.mustNotCall()),