network_admin_notices is a hook triggered to output notices when a user is in the Network admin section (for multi site installs).
<?php add_action('network_admin_notices', callable); ?>
where callable is the function or other callable to be registered.
None
<?php
function my_network_admin_notices() {
echo '<div class="update-nag">Configuration of Foo is not complete!</div>';
}
add_action( 'network_admin_notices', 'my_network_admin_notices' );
?>