Policy - add an explanation for automatic capabilities for transactions and transaction comments

Summary: Ref T7094. I am not sure when this text is legitimately exposed to users - they should be getting an error about not being able to see the object before they get an error about not being able to see a given transaction... That said, I think this text is logically correct at least.

Test Plan: read the text

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7094

Differential Revision: https://secure.phabricator.com/D11632