Model trouble::att is private

[ivmarkov]

The bas_peripheral example has the following comment.

Sounds great - however - and unless I'm missing something - it is currently not possible to handle Connection::EventGatt events directly, because the types necessary to do so (AttReq, AttResp) are not publicly visible (i.e. the trouble::att module is private).

[lulf]

You're right, I think this just needs documenting the new public APIs to be ready.