Add native management (ModelControllerClient) API docs to the Admin Guide

There's some brief docs on configuring the native management interface. There is a separate JIRA for some fundamental docs on jboss-dmr. There will be a page in the "Core management concepts" section on what management resources, attributes and operations are.

This task is to reference those elements and bring together a coherent picture of how to use the ModelControllerClient API.

A subtask would be to add docs on the helper classes in controller-client, although I'm fine with deferring that bit until later in the AS 7.1 cycle,