doc: clarify that fs.Dir async iterator closes automatically

Signed-off-by: James M Snell <jasnell@gmail.com>
Fixes: #35148

PR-URL: #38438
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: jasnell

doc/api/fs.md

@@ -905,6 +905,9 @@ try {
 }
 ```
 
+When using the async iterator, the {fs.Dir} object will be automatically
+closed after the iterator exits.
+
 ### `fsPromises.readdir(path[, options])`
 <!-- YAML
 added: v10.0.0
@@ -5076,6 +5079,9 @@ try {
 }
 ```
 
+When using the async iterator, the {fs.Dir} object will be automatically
+closed after the iterator exits.
+
 #### `dir.close()`
 <!-- YAML
 added: v12.12.0