doc: improved documentation for fs.unlink()

Refs: #11135

PR-URL: #18843
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>

doc/api/fs.md

@@ -2444,8 +2444,21 @@ changes:
 * `callback` {Function}
   * `err` {Error}
 
-Asynchronous unlink(2). No arguments other than a possible exception are given
-to the completion callback.
+Asynchronously removes a file or symbolic link. No arguments other than a
+possible exception are given to the completion callback.
+
+```js
+// Assuming that 'path/file.txt' is a regular file.
+fs.unlink('path/file.txt', (err) => {
+  if (err) throw err;
+  console.log('path/file.txt was deleted');
+});
+```
+
+`fs.unlink()` will not work on a directory, empty or otherwise. To remove a
+directory, use [`fs.rmdir()`][].
+
+See also: unlink(2)
 
 ## fs.unlinkSync(path)
 <!-- YAML
@@ -3173,6 +3186,7 @@ The following constants are meant for use with the [`fs.Stats`][] object's
 [`fs.read()`]: #fs_fs_read_fd_buffer_offset_length_position_callback
 [`fs.readFile()`]: #fs_fs_readfile_path_options_callback
 [`fs.readFileSync()`]: #fs_fs_readfilesync_path_options
+[`fs.rmdir()`]: #fs_fs_rmdir_path_callback
 [`fs.stat()`]: #fs_fs_stat_path_callback
 [`fs.utimes()`]: #fs_fs_utimes_path_atime_mtime_callback
 [`fs.watch()`]: #fs_fs_watch_filename_options_listener