Hi Di,
I don't think there is any such tool which let achieve complete documentation .
few things like dataflow diagram /dataflow objects can be downloaded from metadata repository,similarly transformation can be downloaded as jpg..
you can also go through below
Creating Data Flows or Data Flow Templates - Modeling - SAP Library
Showing and Creating Documentation - Modeling - SAP Library
where as for source object need to maintained manually, overall you can maintain the complete flow in one sheet (excel/word.. etc)