Add a little more guidance to dead code exercise

Taken to the extreme, someone could e.g. just replace the code with
`console.log("HAMSTER: 3, HORSE: 1")`.

Author: illicitonion

Sprint-3/dead-code/exercise-1.js

@@ -1,4 +1,5 @@
 // Find the instances of unreachable and redundant code - remove them!
+// The sayHello function should continue to work for any reasonable input it's given.
 
 let testName = "Jerry";
 const greeting = "hello";

Sprint-3/dead-code/exercise-2.js

@@ -1,4 +1,5 @@
 // Remove the unused code that does not contribute to the final console log
+// The countAndCapitalisePets function should continue to work for any reasonable input it's given, and you shouldn't modify the pets variable.
 
 const pets = ["parrot", "hamster", "horse", "dog", "hamster", "cat", "hamster"];
 const capitalisedPets = pets.map((pet) => pet.toUpperCase());