Docs: Add missing description for $post global.

Props immeet94.
Fixes #46503. See #47110.

git-svn-id: https://develop.svn.wordpress.org/trunk@45742 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -6346,7 +6346,7 @@ function wp_delete_file_from_directory( $file, $directory ) {
  *
  * @since 4.3.0
  *
- * @global WP_Post $post
+ * @global WP_Post $post Global post object.
  */
 function wp_post_preview_js() {
 	global $post;