You need to be modifying a section not a setting.
Try this:
add_action( 'customize_register', function( $wp_customize ) {
$section = $wp_customize->get_section( 'menu_locations' );
$section->description .= "<p>Custom HTML added.</p>";
}, 12 );
The priority of 12
is used because \WP_Customize_Nav_Menus::customize_register()
happens at priority 11
, and this is where the section is added.