Explain what a factorial is and link to wikipedia for more details

Author: mo8it

exercises/18_iterators/iterators4.rs

@@ -1,5 +1,8 @@
 fn factorial(num: u64) -> u64 {
-    // TODO: Complete this function to return the factorial of `num`.
+    // TODO: Complete this function to return the factorial of `num` which is
+    // defined as `1 * 2 * 3 * … * num`.
+    // https://en.wikipedia.org/wiki/Factorial
+    //
     // Do not use:
     // - early returns (using the `return` keyword explicitly)
     // Try not to use: