Update "feature", "branch", and "bookmark" flows to report properly in "arc help"

Summary:
Ref T13490. These workflows are aliases of one another, which is a little silly, but currently all identify as "arc feature" in "arc help".

Straighten that out, at least.

Test Plan: Ran "arc help", "arc branch".

Maniphest Tasks: T13490

Differential Revision: https://secure.phabricator.com/D21086