Slightly update the Diviner documentation

Summary: See PHI1050. Although Diviner hasn't received a ton of new development for a while, it's: not exaclty new; and pretty useful for what we need it for.

Test Plan: Reading.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: leoluk

Differential Revision: https://secure.phabricator.com/D20086