You can’t document each key, but you can tell phpDocumentor what type it is.
You could do something like this:
/**
* Form the array like this:
* <code>
* $array = array(
* 'id' => 'foo', // the id
* 'class' => 'myClass', // the class
* );
*
* </code>
*
* @var array[string]string
*/
$array;