Active effects can be found in artifact.h with short descriptions in comments, but passive effects are only found as a list in artifacts.cpp without any explanation about what they do. Some names are self-explanatory, while others not so much.
The wiki has a page about artifacts and their passive effects that seems to be up to date.
I’ll have to check to know where they are defined in the code, but this will take a little bit of time… I’ll report back with my findings, unless someone else already finds it before me.
Alright, I’ve checked - and the effects are described in enums.h (enum art_effect_passive).
As for the messages that the player recieves, they can be found in game.cpp (function game::add_artifact_messages(…)).
Nah, it is not up to date. There are more effects defined in the code than wiki lists, both passive and active.
Indeed, it seems like it has gained
AEP_FUN, // Slight passive morale.