Note that `arc land` supports Mercurial in arc docs

Summary: Also mentioned a few other features which have come into existence since this was last updated. Ref T2922.

Test Plan: Proofread.

Reviewers: DurhamGoode, btrahan

Reviewed By: btrahan

CC: adityar7, aran

Maniphest Tasks: T2922

Differential Revision: https://secure.phabricator.com/D5628