Document checkboxes and radios element can have individual descriptions

Checkboxes and radios form elements can both have descriptions added to each item (or indeed other specific form properties). However, this is not documented and not widely known.

It is however covered by our tests, in FormTestCheckboxesRadiosForm:

    // Expand #type checkboxes, setting custom element properties for some but not
    // all options.
    $form['checkboxes'] = array(
      '#type' => 'checkboxes',
      '#title' => 'Checkboxes',
      '#options' => array(
        0 => 'Zero',
        'foo' => 'Foo',
        1 => 'One',
        'bar' => $this->t('<em>Bar - checkboxes</em>'),
        '>' => "<em>Special Char</em><script>alert('checkboxes');</script>",
      ),
    );
    if ($customize) {
      $form['checkboxes'] += array(
        'foo' => array(
          '#description' => 'Enable to foo.',
        ),
        1 => array(
          '#weight' => 10,
        ),
      );
    }

The slightly clearer way to do this is as follows:

    $form['checkboxes'] = array(
      '#type' => 'checkboxes',
      '#title' => 'Checkboxes',
      '#options' => array(
        'foo' => 'Foo',
        'bar' => 'Bar',
     );
   $form['checkboxes']['foo']['#description'] = 'description for the foo checkbox';

Documentation should be added to the Checkboxes and Radios element classes.