Docs: Add a @global entry for $post variable in wp_post_preview_js().

Props avinapatel.
Fixes #41194.

git-svn-id: https://develop.svn.wordpress.org/trunk@40959 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -5455,6 +5455,8 @@ function wp_delete_file( $file ) {
  * This prevents reusing the same tab for a preview when the user has navigated away.
  *
  * @since 4.3.0
+ *
+ * @global WP_Post $post
  */
 function wp_post_preview_js() {
 	global $post;