WordPress at a glance
action is not described

wp_upgrade action-hook . WP 3.9.0

Fires after a site is fully upgraded.

Usage

add_action( 'wp_upgrade', 'action_function_name_3779', 10, 2 );
function action_function_name_3779( $wp_db_version, $wp_current_db_version ){
	// action...
}
$wp_db_version(int)
The new $wp_db_version.
$wp_current_db_version(int)
The old (current) $wp_db_version.

Where the hook is called

wp_upgrade()
wp_upgrade
wp-admin/includes/upgrade.php 524
do_action( 'wp_upgrade', $wp_db_version, $wp_current_db_version );

Where the hook is used (in WP core)

Does not used.