Fix wrong documentation on \Drupal\migrate\Plugin\migrate\source\SourcePluginBase::initializeIterator()

Problem/Motivation

In \Drupal\migrate\Plugin\migrate\source\SourcePluginBase::initializeIterator() the docblock states:

  /**
   * Initializes the iterator with the source data.
   *
   * @return array
   *   An array of the data for this source.
   */
  abstract protected function initializeIterator();

However this is incompatible with the iterator_count($this->initializeIterator()) that happens some lines below in the same file.

Proposed resolution

Change the docblock to document that the method should return an istance of \Traversable (either \Iterator or \IteratorAggregate).

Remaining tasks

User interface changes

API changes

Data model changes