Missing title_prefix, title_suffix, attributes available variables for aggregator twig files

Someone needs to review this carefully. I am not sure whether:
- these are actually present (I didn't look through the code to figure this out)
- if we normally document them. They look kind of like standard attributes that are present on pretty much every Twig file, so I am not sure we document them in every Twig file?

Hopefully the Twig maintainers will figure this out... moving to theme component since it needs some expert reviewing.

Yes we usually document the variables that we use in the template and they are in the templates.

The work that needs to be done is for this to be added to each template in classy/stable too.

Also they are the magic ones that get added to every template via _template_preprocess_default_variables().

Please ensure the text for the documentation is the same as for those same variables, elsewhere in core.

I added missing variables documentation to these files:

core\themes\classy\templates\content\aggregator-item.html.twig
core\themes\classy\templates\dataset\aggregator-feed.html.twig
core\themes\stable\templates\content\aggregator-item.html.twig
core\themes\stable\templates\dataset\aggregator-feed.html.twig

There is still variable 'full' that is not documented. The value is assigned in core\modules\aggregator\aggregator.theme.inc (line 48):

function template_preprocess_aggregator_feed(&$variables) {
...
$variables['full'] = $variables['elements']['#view_mode'] == 'full';
...
}

Cleaning up drupalcampfi tags.

I reviewed 2607548-8.patch: the title variables have been updated, plus documentation for those variables, in core, classy, and stable template files for the aggregator module.

Committed/pushed to 8.1.x and 8.2.x, thanks!