getDocumentById   
  @GET(value = "documents/{id}") 
Get a document This endpoint will retrieve the details of a single document with the given ID. Responses:
- 200: Success. The document was successfully retrieved. 
- 400: Bad Request. This will occur if you provide an invalid value for a request parameter. 
- 401: Unauthorized. This will occur if you do not supply an access-token to this endpoint, or if you provide an expired or invalid access token. 
- 403: Forbidden. This will occur if you try to use this endpoint without being setup for documents. 
- 404: Not Found. The document ID you have provided is not one of this user's documents. 
- 500: Internal Server Error. 
Return
Parameters
id
A unique Id that identifies this document in the Sensibill system.