Make more explicit what the operations array index passed to batch_set() must contain

The correct code that uses batch_set() is similar to the following one. (It has been taken from node_mass_update().)

    $batch = array(
      'operations' => array(array('_node_mass_update_batch_process', array($nodes, $updates))),
      'finished' => '_node_mass_update_batch_finished',
      'title' => t('Processing'),
      // We use a single multi-pass operation, so the default
      // 'Remaining x of y operations' message will be confusing here.
      'progress_message' => '',
      'error_message' => t('The update has encountered an error.'),
      // The operations do not live in the .module file, so we need to
      // tell the batch engine which file to load before calling them.
      'file' => drupal_get_path('module', 'node') . '/node.admin.inc',
    );
    batch_set($batch);

Notice how the 'operations' array index is defined, in particular the array($nodes, $updates) part.
If you replace that with array($nodes, 'updates' => $updates), you will get an error message described in the issue summary.

batch_set() describes 'operations' as:

operations: (required) Array of operations to be performed, where each item is an array consisting of the name of an implementation of callback_batch_operation() and an array of parameter.

The example shown does not use an associate array, but the documentation does not say an associative array must not be used. That should be probably made explicit.