Undocumented title variable in feed-icon.html.twig

Problem/Motivation

The feed-icon.html.twig template uses a top-level variable called title. This is originally defined in core/includes/theme.inc:

'feed_icon' => [
   'variables' => ['url' => NULL, 'title' => NULL],
 ],

It isn't mentioned in the doc-block at the start of feed-icon.html.twig though.

Proposed resolution

Document the top-level title variable in the feed-icon.html.twig templates.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet