Docs: Standardize the file header summary for wp-admin/includes/file.php.

Also adds an `@since` version and corrects the `@subpackage` value.

See #33701.


git-svn-id: https://develop.svn.wordpress.org/trunk@35221 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/file.php

@@ -1,11 +1,14 @@
 <?php
 /**
+ * Filesystem API: Top-level functionality
+ *
  * Functions for reading, writing, modifying, and deleting files on the file system.
  * Includes functionality for theme-specific files as well as operations for uploading,
  * archiving, and rendering output when necessary.
  *
  * @package WordPress
- * @subpackage Administration
+ * @subpackage Filesystem
+ * @since 2.3.0
  */
 
 /** The descriptions for theme files. */