Document how to deprecate Twig variables

Made an attempt: https://www.drupal.org/node/2856615/revisions →

Thanks, I did some further fixes to the order, formatting and sample code, leaving at NR.

Latest revision looks good, the document can be updated with the same. Hence marking this RTBC

Yes let's close this one, we can always tweak further.

Automatically closed - issue fixed for 2 weeks with no activity.