Edited docs to improve context

Updated versioning, added core concept definitions, modified secret description.
Moved the Secrets and Orders Examples to after the API operations info.
Added Limits section for traceability of current limits.
Removed Architecture and Service Endpoints sections as these are not
specified in other OpenStack docs or relevant to non-RAX deployments.
Made capitalization consistent for sections.

Change-Id: I45dabb16299879f8580efa5878e21e6aa0239b8e
Merge "Edited docs to improve context"