Flesh out for loop example (google#1841)

* Add example of iterating over a collection.
* Update speaker notes to call out the use of iterators.

I think it's useful to call out that `for` loops primarily are used to
iterate over a collection of objects, even though we haven't yet talked
about any concrete collection types at this point. I think using the
array literal syntax is simple enough to understand that it should be
quick to explain when we get to this slide.

src/control-flow-basics/loops/for.md

@@ -1,19 +1,25 @@
 # `for`
 
 The [`for` loop](https://doc.rust-lang.org/std/keyword.for.html) iterates over
-ranges of values:
+ranges of values or the items in a collection:
 
 ```rust,editable
 fn main() {
     for x in 1..5 {
         println!("x: {x}");
     }
+
+    for elem in [1, 2, 3, 4, 5] {
+        println!("elem: {elem}");
+    }
 }
 ```
 
 <details>
 
-- We will discuss iteration later; for now, just stick to range expressions.
+- Under the hood `for` loops use a concept called "iterators" to handle
+  iterating over different kinds of ranges/collections. Iterators will be
+  discussed in more detail later.
 - Note that the `for` loop only iterates to `4`. Show the `1..=5` syntax for an
   inclusive range.