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

See #54729, #55499.
Built from https://develop.svn.wordpress.org/trunk@53060


git-svn-id: http://core.svn.wordpress.org/trunk@52649 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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.