[bitboard] Add a doc comment to BitBoard::first_occupied_square

bitboard/src/bitboard.rs

@@ -287,6 +287,12 @@ impl BitBoard {
         }
     }
 
+    /// Get the first occupied square in the given direction.
+    ///
+    /// ## To-Do
+    ///
+    /// - Take `direction` by value instead of reference
+    ///
     #[must_use]
     pub fn first_occupied_square(&self, direction: &IterationDirection) -> Option<Square> {
         match direction {