finagle-doc: Updated tunables documentation
finagle-doc: Updated tunables documentation

Summary: Problem

As Tunable users, we have found that the existing documentation was not enough
to easily understand what they are and how they should be used.

Solution

Add more context to the docs.

Differential Revision: https://phabricator.twitter.biz/D116301

luciferous/finagle,twitter/finagle,luciferous/finagle,mkhq/finagle,luciferous/finagle,mkhq/finagle,luciferous/finagle,twitter/finagle,mkhq/finagle,luciferous/finagle,mkhq/finagle,mkhq/finagle,twitter/finagle,twitter/finagle,luciferous/finagle,mkhq/finagle,twitter/finagle