Fix some typos in the new cluster docs

Summary:
While reading the new cluster docs, I noticed a few minor typos, and one
section that seemed to be incomplete and redundant, so I just removed it.

Test Plan: none.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: chad, Korvin, jshirley

Differential Revision: https://secure.phabricator.com/D15704