Fix engine docs

Cargo.toml

@@ -8,7 +8,7 @@ license = "MIT"
 keywords = ["glicko-2", "glicko", "rating", "elo"]
 categories = ["game-development", "mathematics"]
 description = "Implementation of the Glicko-2 rating system that allows for instant feedback after games, not just once a rating period closes"
-# rust-version = # TODO
+# TODO: rust-version =
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 

src/engine.rs

@@ -408,7 +408,10 @@ impl RatingEngine {
         self.player_rating_at(player, Instant::now())
     }
 
-    /// Returns a player
+    /// Calculates a player's rating at a given point in time.
+    /// The calculation is based on the registered results for this player (see [`register_result`][Self::register_result]).
+    /// Note that this function is not cheap.
+    /// The rating deviation of this result also depends on the current time, because rating deviation increases with time.
     ///
     /// This function is meant mostly for testability.
     ///