Address #293:  document that app code should not pass 'dataset_id' to 'Key.__init__'.

Only the 'helpers.key_from_protobuf' factory should pass it.

Fixes #293.
s/helprs/helpers

I also wanted to include a longer discussion in the docstring (not in the param description) about cases where they can expect the value to be set / cases when they can't and to briefly explain that ` Dataset` / `Connection` pair will get the dataset ID correct.