Document the need for 's3a' when doing model exporting to s3

If user wants to export a model to s3 using `save_model`, the first thing they try is an `s3` uri which raises a not implemented error. Instead they need to use `s3a`, which isn't obvious (at least to me).

Suggest we add an example and note to: http://docs.h2o.ai/h2o/latest-stable/h2o-docs/save-and-load-model.html

Might be worth changing the error message to suggest using `s3a`.

I didn't test it, but we should also document the requirements for `load_model`.