add some documentation

src/artifacts.rs

@@ -9,6 +9,7 @@ pub trait Artifact {
     fn get_immutable_position(&self) -> &Position;
     /// call to apply the effects of the artifact to the player
     fn collect(&mut self, player: &mut Player);
+    /// returns if the artifact was collected and can be removed from the level
     fn was_collected(&self) -> bool;
 }