Ensure API endpoints deployment via WSGI are documented

Governance community goal for Pike: Control Plane API endpoints deployment via WSGI

In some projects, API services can be run:

1. As a Python command that runs a web server (e.g. wsgiref or werkzeug).
2. As a WSGI application hosted by performant web servers (e.g. Apache with mod_wsgi or Nginx with uWSGI).

When a project provides a WSGI application the API service gains flexibility in terms of deployment, performance, configuration and scaling.

See here for more info: https://github.com/openstack/governance/blob/master/goals/pike/deploy-api-in-wsgi.rst