circe-format-server-rejoin: Document more format parameters.

These were added in the recent rewrite, but not documented in this
docstring.
circe-channel-rename-user: Rewrite to be more clear.

The previous implementation had some weird corner cases which I
did not understand, the new implementation should be a lot more clear.
The function should also only be called in channels where the renaming
user is actually present.

Also remove the outdated change to user info in display-NICK.