Add doc-tests to rotation_construction.

src/geometry/rotation_construction.rs

@@ -14,6 +14,16 @@ where
     DefaultAllocator: Allocator<N, D, D>,
 {
     /// Creates a new square identity rotation of the given `dimension`.
+    ///
+    /// # Example
+    /// ```
+    /// # use nalgebra::Quaternion;
+    /// let rot1 = Quaternion::identity();
+    /// let rot2 = Quaternion::new(1.0, 2.0, 3.0, 4.0);
+    ///
+    /// assert_eq!(rot1 * rot2, rot2);
+    /// assert_eq!(rot2 * rot1, rot2);
+    /// ```
     #[inline]
     pub fn identity() -> Rotation<N, D> {
         Self::from_matrix_unchecked(MatrixN::<N, D>::identity())