Document why "accept" is sticky

Summary: This is a fairly common question but I think it's the right product
behavior, document it so I can reference the docs next time it comes up.

Test Plan: Generated and read documentation.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T735

Differential Revision: https://secure.phabricator.com/D1310