Don't close revisions on "arc amend"

Summary:
Ref T4603. This workflow predates `arc land` and doesn't make much sense in modern Phabricator/Arcanist. It is surprising that `arc amend` will sometimes close accepted revisions, and we're better at detecting that repositories are tracked, and tracking repositories is easier.

Also fix some inaccuracies and old claims in the documenation and help.

Test Plan: Ran `arc amend`.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4603

Differential Revision: https://secure.phabricator.com/D8522