I couldn’t find a hook so I went with the JS option.
add_action( "admin_head-post-new.php", 'meta_box_instruction' ); //new post
add_action( "admin_head-post.php", 'meta_box_instruction' ); //edit post
function meta_box_instruction($d) {
global $post;
if($post->post_type == '{YOUR POST TYPE}' || $_GET['post_type'] == '{YOUR POST TYPE}'):
?>
<script type="text/javascript">
jQuery(document).ready(function(){
jQuery('<p>Some instruction</p>').insertBefore('#taxonomy-{YOUR TAXONOMY NAME}').parent();
});
</script>
<?php
endif;
}