Correct references of @uses $wpdb in core documentation to use @global.

See #30191, [30105].
Fixes #30217.


git-svn-id: https://develop.svn.wordpress.org/trunk@30122 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/capabilities.php

@@ -104,7 +104,8 @@ class WP_Roles {
 	 *
 	 * @since 2.1.0
 	 * @access protected
-	 * @uses $wpdb Used to get the database prefix.
+	 *
+	 * @global wpdb  $wpdb          WordPress database abstraction object.
 	 * @global array $wp_user_roles Used to set the 'roles' property value.
 	 */
 	protected function _init() {