Fix references in docs

[Jakuje]

Originally posted by @wiktor-k in #255 (comment)

I wonder why this line couldn't just be:
/// See [Session::encrypt_message].
Is it because Session is not in scope? (then it can be imported above with #[cfg(doc)] attribute) (we do something similar in signstar)

[Jakuje]

This was solved only partially by #334. There are still few cases unresolved as mentioned in #334 (comment):

And I think we should change this also in the other cases, like encrypt_message, encapsulate, ... that were added more recently.

[hug-dev]

yes missed that sorry!