Put message in ACF interface to warn people to edit custom fields in the PHP

add-warning-to-acf.php

<?php
// put message in ACF interface to warn people to edit custom fields here
function acf_warning( $hook ) {
    if ('edit.php' != $hook) {
        return;
    }

    $javascript = "
        var acfFieldGroupEl = document.getElementById('acf-field-group-wrap');

        if (acfFieldGroupEl) {
            var warningEl = document.createElement('h3');
            warningEl.innerHTML = 'WARNING: Custom Field definitions are in " . basename(__FILE__) . "';
            warningEl.style.color = 'red';
            acfFieldGroupEl.insertBefore(warningEl, acfFieldGroupEl.firstChild);
        }
    ";

    // needed because Wordpress is weird
    wp_enqueue_script('acf-warning-js', '/iAmNotReal.js', array(), '1.0', true);

    if(function_exists("wp_add_inline_script")){
        wp_add_inline_script('acf-warning-js', $javascript);
    }
}

add_action('admin_enqueue_scripts', 'acf_warning');