How can I add text to a specific ‘Edit Page’?

To do this you would indeed have to include some code in the functions.php of your theme. You can use admin_notices to add a note to this specific page like this:

add_action( 'admin_notices', 'wpse332074_donotedit' );
function wpse332074_donotedit () {
 $screen = get_current_screen();
 if ($screen->post_type == 'page') {
   $variable = $_GET['post'];
   if ($variable == id_of_page_as_integer) { 
     $class="notice notice-error";
     $message = __( 'Do not edit this page!', 'your-text-domain' );
     echo '<div class="' . $class . '"><p>' . $message . '</p></div>';
     }
   }
 }

However, this would allow people to ignore the message and still edit the page. If you really don’t want the page to be edited, you can remove the edit, quick edit and trash links in the all pages screen by using the page_row_actions filter:

add_filter( 'page_row_actions', 'wpse332074_disable_edit', 10, 2 );
function wpse332074_disable_edit ($actions, $post) {
  if ( $post->ID == id_of_page_as_integer ) {
    unset( $actions['edit'] );
    unset( $actions['inline hide-if-no-js'] );
    unset( $actions['trash'] );
    }
  return $actions;
  }

Beware that any changes to functions.php will be lost when the theme is updated. The proper way to do this is to build a child theme.