Update arcanist documentation to reflect "land", a sane relative commit, and "--auto"

Summary: See D2080. The introduction of arc land, defaulting to origin/master, and --auto enormously simplifies the documentation.

Test Plan: Read documentation.

Reviewers: btrahan

Reviewed By: btrahan

CC: 20after4, aran

Maniphest Tasks: T894

Differential Revision: https://secure.phabricator.com/D2082