[board] Add a few comments in position::position

One TODO
And one doc comment.

board/src/position/position.rs

@@ -89,9 +89,12 @@ impl Position {
             return false;
         }
 
+        // TODO: Perform a real check that the player can castle.
+
         true
     }
 
+    /// Return a PlacedPiece representing the rook to use for a castling move.
     pub(crate) fn rook_for_castle(&self, player: Color, castle: Castle) -> Option<PlacedPiece> {
         let square = match (player, castle) {
             (Color::White, Castle::KingSide) => Square::H1,