Adds a hint under the "Username" field at checkout.

add_username_hint_at_checkout.php

<?php

// Copy from below here...

/*
 * Adds a hint under the "Username" field at checkout.
 */
function my_pmpro_checkout_add_username_hint() {
	?>
	<script>
		jQuery("#username").parent().append("<p><small class='lite'>Usernames cannot be changed after your account is created.</small></p>");
	</script>
	<?php
}
add_action( 'pmpro_checkout_after_username', 'my_pmpro_checkout_add_username_hint' );