Customizer: How do you add HTML to control labels?

You should use CSS for this. For example:

#customize-control-display_about_text label {
    font-weight: bold;
}

Enqueue the stylesheet for this at the customize_controls_enqueue_scripts action.

If you must use JS to modify a control, then note that I would not advise using jQuery.ready in the other answer. You should instead make use of the Customizer JS API to access the control once it is ready, for example enqueue a script at the customize_controls_enqueue_scripts action which contains:

wp.customize.control( 'display_about_text', function( control ) {
    control.deferred.embedded.done( function() {
        control.container.find( 'label' ).wrapInner( '<strong></strong>' );
    });
});

This pattern can be seen in core actually in how the textarea for the Custom CSS feature is extended: https://github.com/WordPress/wordpress-develop/blob/4.8.0/src/wp-admin/js/customize-controls.js#L5343-L5346