Docs: Add missing descriptions for the $wpdb global in DocBlocks all the places.

See #32246.

Built from https://develop.svn.wordpress.org/trunk@35170


git-svn-id: http://core.svn.wordpress.org/trunk@35136 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/revision.php

@@ -582,7 +582,7 @@ function _wp_get_post_revision_version( $revision ) {
  * @since 3.6.0
  * @access private
  *
- * @global wpdb $wpdb
+ * @global wpdb $wpdb WordPress database abstraction object.
  *
  * @param WP_Post $post      Post object
  * @param array   $revisions Current revisions of the post