add back documentation of `timeit`

src/lib.rs

@@ -57,6 +57,12 @@ where
     );
 }
 
+/// measure the time it takes to do something
+///
+/// # Example
+/// ```rust
+/// let t = timeit(|| 1 + 2);
+/// ```
 pub fn timeit<F, O>(mut f: F) -> Duration
 where
     F: FnMut() -> O,