doc: improved documentation for fs.unlink()

doc/api/fs.md

@@ -2749,8 +2749,27 @@ 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()`.
+
+```js
+// Assuming that 'tmpDir' is a directory.
+fs.unlink('tmpDir', (err) => {
+  // err is not null because a directory cannot be deleted through unlink().
+});
+```
+
+See also: unlink(2)
 
 ## fs.unlinkSync(path)
 <!-- YAML