[Doc] Mention that retrieve is lazy and doesn't make any HTTP call

[maingoh]

This is the hack I currently use to infere on resource without making an API call to retrieve the data :

from deepomatic.resources.recognition import RecognitionVersion

version = RecognitionVersion(client.RecognitionVersion._helper, pk=version_id)

task = version.inference(...)

[maingoh]

I was confused by the documentation, it appears retrieving the data is lazy. We should mention it and fix the sentence in the demo

 - retrieve(id):  allow to retrieve the JSON data representing the resource with given ID.