Adds serialized response formats to End User Guide

With the moving of the Object Storage API content from a
long-form dev guide to a specification, some topics needed
to be added to the End User Guide. Also included linked
environment variables topic.

Change-Id: I6b8cee64e0bb7ea8ba7291aea31b4b7853f214a3
Partial-bug: 1392382
Merge "Adds serialized response formats to End User Guide"