Update "Autoclose" documentation to focus on "Permanent Refs" instead

Summary:
Depends on D20433. Ref T13277. Since "Autoclose" no longer exists, update the documentation.

Currently, this documentation focuses a lot on troubleshooting because users historically had a lot of trouble with figuring out why things were or were not autoclosing. I haven't seen any real confusion about this in years, so I suspect we may have improved the import pipeline and/or UI to make this less of a problem.

It's also possible that this document "fixed" the problem, but usually I expect a documentation fix to not affect the frequency of reports, just make them easier to resolve, so I doubt it.

If unclear things remain and documentation really did fix it, maybe we can fix the issues. Or we can just put the troubleshooting documentation back.

Test Plan: Read documentation.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13277

Differential Revision: https://secure.phabricator.com/D20434