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-admin/post.php

@@ -30,7 +30,7 @@ $post_ID = $post_id;
 /**
  * @global string  $post_type
  * @global object  $post_type_object
- * @global WP_Post $post
+ * @global WP_Post $post             Global post object.
  */
 global $post_type, $post_type_object, $post;