Issue #3307468 by darvanen: Improve Batch API $context['sandbox'] documentation

 *     $context['sandbox'] will be there the next time this function is called

 *     for the current operation. For example, an operation may wish to store a

 *     pointer in a file or an offset for a large query. The 'sandbox' array key

 *     is not initially set when this callback is first called, which makes it

 *     useful for determining whether it is the first call of the callback or

 *     not:

 *     is empty when this callback is first called, which makes it useful for

 *     determining whether it is the first call of the callback or not:

 *     @code

 *       if (empty($context['sandbox'])) {

 *         // Perform set-up steps here.