Document changes in authentication for Istio adapter

In the current version (TP7) the only supported authentication option is API key passing a query param. This is going to change in TP8 (Feb 20th).

Current documentation where changes will be needed:
https://docs.openshift.com/container-platform/3.11/servicemesh-install/servicemesh-install.html#authenticating-requests

In TP8, we're planning on supporting:

• User Key and App ID authentication
• The ability to read from query params or headers
• The ability to set query params or headers label values

Configuration will be on the custom resource on OpenShift (no UI in 3scale). This part should be added to the docs.