Make parameters more obvious

[sgwilym]

I wish the web page came out and said what Willow actually provides and what is up to the developer.

We do already have the explicit parameter lists, so not quite sure this is necessary. Maybe we could emphasize that (unlike, say, noise) we only leave open extremely boring stuff like "which hash function is considered secure in the year 2024", not "which pieces of information should a handshake contain and how should they be responded to".