added doc / Update robot.rs

src/robot.rs

@@ -56,6 +56,7 @@ pub enum Orientation {
 }
 
 impl Distribution<Orientation> for Standard {
+    /// Generating random orientation.
     fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> Orientation {
         match rng.gen_range(0, 3) {
             0 => Orientation::N,