Change callback cumulativeCounter suggested use

Summary:
I did not like the old way of doing things.  I think this is
a better way to have people interact with a cumulative counter that
uses a callback.

Also updated docs

Test Plan: extended the unit tests

Reviewers: rajiv, kris

Reviewed By: kris

Subscribers: Diffs_emails

Differential Revision: http://phabricator.signalfuse.com/D2653