Docs: Add missing description for $pagenow global in various functions.

See #54729, #55499.

git-svn-id: https://develop.svn.wordpress.org/trunk@53060 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/post.php

@@ -6577,7 +6577,7 @@ function wp_update_attachment_metadata( $attachment_id, $data ) {
  *
  * @since 2.1.0
  *
- * @global string $pagenow
+ * @global string $pagenow The filename of the current screen.
  *
  * @param int $attachment_id Optional. Attachment post ID. Defaults to global $post.
  * @return string|false Attachment URL, otherwise false.