Change Job Description field

// Add your own function to filter the fields
add_filter( 'submit_job_form_fields', 'custom_submit_job_form_fields' );

// This is your function which takes the fields, modifies them, and returns them
// You can see the fields which can be changed here: https://github.com/mikejolley/WP-Job-Manager/blob/master/includes/forms/class-wp-job-manager-form-submit-job.php
function custom_submit_job_form_fields( $fields ) {

    // Here we target one of the job fields (job_title) and change it's label
    $fields['job']['job_description']['description'] = "Note: Use headings to make your advert stand out. To see example headings and listing <a href='http://example.com'>click here</a>.";

    // And return the modified fields
    return $fields;
}