attributes: {
    description: {
        type: "text",
        // no source, so it goes straight into the html comment, nice for dynamic rendering
    }
}


...
"render_callback" => function( $attributes ) {
    return "<p>" . $attributes["description"] . "</p>"
}


// var_dump($attributes["description"]
array(3) {
    [0]=>
    string(24) "Rendering RichText with "
    [1]=>
    array(5) {
      ["type"]=>
      string(6) "strong"
      ["key"]=>
      string(10) "_domReact4"
      ["ref"]=>
      NULL
      ["props"]=>
      array(1) {
        ["children"]=>
        string(3) "php"
      }
      ["_ower"]=>
      NULL
    }
    [2]=>
    string(22) " is a pain in the a..."
  }