Attempt to document usage of ref in azip
///
/// type M = Array2<f32>;
///
+/// // Since this function borrows its inputs, captures must use ref
+/// fn borrow_multiply(a: &mut M, b: &M, c: &M) {
+/// azip!(mut a (a), ref b, ref c in { *a = b * c });
+/// }
+///
/// fn main() {
/// let mut a = M::zeros((16, 16));
/// let mut b = M::zeros(a.dim());
/// });
///
/// assert_eq!(a, &b - &c);
+///
+/// borrow_multiply(&mut a, &b, &c);
+///
+/// assert_eq!(a, &b * &c);
/// }
/// ```
macro_rules! azip {
