Document /policies endpoint for APICAST_MANAGEMENT_API env var and Management API

Current state:

Since 3scale 2.6 the endpoint /policies is available for the Management API, however it is only documented in the GH repository in APIcast parameters. 

The /policies endpoint is not documented in the following documentation: 

• Our official documentation, APICAST_MANAGEMENT_API.
• Management API document in the GH repo, which is linked in the parameters page and our official documentation.

Desired state, questions:

• Update the official documentation and the Management API document with the policies endpoint OR remove it from the APIcast parameters.
  • Should the /policies endpoint be documented? When using the /policies endpoint in APIcast self-managed the probes are failing since the /status endpoint is not enabled.
  • What is the purpose of the /policies endpoint if it can be called when the /status endpoint is enabled?