Inline docs for BatchAPI

1. It's not clear where the params for the finished callback batch_example_finished come from:

function batch_example_finished($success, $results, $operations) {

Op 1 has this:

  // Store some result for post-processing in the finished callback.
  $context['results'][] = $node->nid . ' : ' . check_plain($node->title);

Is that it? If so, there's also surely a bug in the code as op 2 doesn't put anything into the results key.

2. Where does the progress message get its placeholders from?

    'progress_message' => t('Processed @current out of @total.'),

I can't see anything being put into $context that looks remotely like either of those!