This is an adaption of an answer by /user/gmazzap so there may be better way to go about it. Either way, I got it to work.
Pretty much you would filter the checklist arguments and pass in a new Walker Class which you can then define what happens at the start of the list start_lvl(). At that point you should be able to pass any HTML you need to. You should test this in a few cases just to make sure it works in the scenario(s) you need it to. Also note that there’s a taxonomy limit on the conditional in taxonomy_checklist_desc() so that it only shows in category but you can remove this to apply to all taxonomies or add more taxonomies if need be.
/**
* Add Description To Taxonomy Term Checklist Metabox
* @param array $args
* @return array $args
*/
function taxonomy_checklist_desc( $args ) {
if (
! empty( $args['taxonomy'] )
&& ( $args['taxonomy'] === 'category' ) // only for 'category' taxonomy
&& ( ! isset( $args['walker'] ) || ! $args['walker'] instanceof Walker )
) {
$args['walker'] = get_Walker_Tax_Meta_Desc();
}
return $args;
}
function get_Walker_Tax_Meta_Desc() {
class Walker_Tax_Meta_Desc extends Walker_Category_Checklist {
function start_lvl( &$output, $depth = 0, $args = array() ) {
?>
<a href="http://i.imgur.com/eu6xBup.jpg"><em>This is where I'd put my description... IF I HAD ONE!</em></a>
<?php
parent::start_lvl( $output, $depth = 0, $args );
}
}
return new Walker_Tax_Meta_Desc;
}
add_filter( 'wp_terms_checklist_args', 'taxonomy_checklist_desc' );