R1:d4b3cd5255b6

Document the "bin/auth revoke" tool

Summary: Depends on D18910. Ref T13043. Provides reasonable user-facing documentation about the general role and utility of this tool.

Test Plan: Read document.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13043

Differential Revision: https://secure.phabricator.com/D18911