Improve documentation on Plugin Outputs

[benstov]

I have just noticed this in the documentation, I don't get exactly how the FUNCTION_ENDPOINT supposed to get its value.

module:
  description: Hello world container service
  type: container
  name: hello-container
  services:
    ...
      dependencies:
        - hello-function
      env:
        FUNCTION_ENDPOINT: ${modules.hello-function.outputs.endpoint}

[edvald]

Agreed. The plan is to explicitly define the schema for outputs at the plugin level, so that those docs can be generated automatically.