This forum is suitable for one-off reports like this; the document you are referring to is publicly available, so it may be that other users will encounter the same discrepancy and wonder what the right answer is.
The error that you refer to is actually present in two locations in the guide, both for the gatts_notify_handle and gatts_indicate_handle API commands. The correct data type for the data argument is uint8a, as you noted. This will be fixed in the *D revision of the User Guide. The host API library code is right.
I'm glad to hear the otherwise positive feedback on the host library and User Guide! Hopefully you don't encounter other errors, but please follow up here if you do.