Clarify launch_onion_service_with_hsid behavior

The following discussion from !2402 (merged) should be addressed:

• @opara started a discussion: (+3 comments)

  Is it worth adding a comment to this method saying that the keys may be written to disk? If I was seeing this API for the first time, I would probably think that I was making an ephemeral onion service. I think it might be unexpected to people unfamiliar with arti internals that the keys given in this method will be saved in persistent storage, and doing something unexpected with a key might be worth a comment.