modify text at the bottom of excerpt editor

Try below code.

add_filter( 'gettext', 'excerpt_gettext', 10, 2 );
function excerpt_gettext( $translation, $original ) {
 $post_type = get_post_type( $_GET['post'] );
 if($post_type == 'your custom post type') {
   $text = strpos($original, 'Excerpts are optional hand-crafted summaries of your content that can be used in your theme');
   if ($text !== false) {
     return  'My Custom Excerpt Discription';
   }
 }
 return $translation;
}

NOTE: Change “your custom post type” with your actual post type name

Hope this will helps you