util/doc: Document full histogram details
util/doc: Document full histogram details

Summary: Problem / Solution

While we have a section about finagle-stats, we do
not mention that users can see the full histogram
details on the /admin/histogram admin page.

JIRA Issues: csl-2945

Differential Revision: https://phabricator.twitter.biz/D143293

twitter/util,twitter/util,twitter/util