FormAPI docs is missing info on subarrays in select #options

Apparently you can group options together with headings in a SELECT form element. This is not mentioned at all in the Form API reference document.

Here's an example of an array that works:

  $options = array(
    'Group 1' => array(
      'a' => t('Option A'),
      'b' => t('Option B'),
    ),
    'Group 2' => array(
      'x' => t('Option X'),
      'y' => t('Option Y'),
    ),
  );

Headings appear in bold italic and are not selectable by the user.

Details here: http://drupal.org/node/76500#comment-3451808