Add speaker note regarding passing slices instead

This section covers both references and slices, but the exercise focuses
on references. This speaker note briefly discusses the option to use
slices instead, and a cost of doing so.

Author: djmitche

src/references/solution.md

@@ -10,4 +10,9 @@
   element. This is because we're iterating using a mutable reference to an
   array, which causes the `for` loop to give mutable references to each element.
 
+- It is also possible to take slice references here, e.g.,
+  `fn
+  magnitude(vector: &[f64]) -> f64`. This makes the function more general,
+  at the cost of a runtime length check.
+
 </details>