Removed documentation of the "highlighted_fields" key from result set extra data

\Drupal\search_api\Query\ResultSetInterface::getExtraData() previously documented the highlighted_fields as a way of specifying highlighted fields data for the result set.

However, the same key was also documented, for the same purpose, on result items, via \Drupal\search_api\Item\ItemInterface::getExtraData(), which is actually the proper way to set highlighting data. The key was therefore dropped from the documentation of ResultSetInterface::getExtraData().

Since the "Highlight" processor previously used the wrong method (setting the data on the result set, not the items), it was updated, too, to use the new standard mechanism.