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