Missing documentation #autocomplete_route_name on Form API

This file is in the Documentation project git repository. It is nearly impossible to edit. Sorry. It is really not updated at all for Drupal 8.

Instead of editing this file at all, we're trying to get rid of it and replace with maintainable documentation:
#1617948: [policy for now] New standard for documenting form/render elements and properties

The Form API reference is going away for Drupal 8. See
#100680-118: [meta] Make API module generate Form API documentation
for details on what will replace it (basically, there will soon be an Elements page on api.drupal.org, linking to the form/render plugins).

So we are now documenting form/render elements and their specific properties on the element classes. See #2486967: [meta] Move/Create Form Element Documentation
and child issues.

The common properties for all form and all render elements are being documented on the base form/render element classes. This is issue
#2599442: Document common form/render element properties
That patch takes care of the #autocomplete_route_name property.