Clarify usage of return values in iterator protocol (mdn#37511)

* Clarify usage of return values in iterator protocol

There's an example with an iterable that returns an iterator that enumerates 1, 2, and returns with `{done: true, value: 3}`, and is evaluated with `const [b, c, d] = obj`.

It takes a careful reading to notice that the third value is used only to ensure that the iterator is exhausted and that the value from the iterator return is not bound to `d`.

* Update files/en-us/web/javascript/reference/iteration_protocols/index.md

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Co-authored-by: Joshua Chen <[email protected]>

files/en-us/web/javascript/reference/iteration_protocols/index.md

@@ -225,6 +225,7 @@ const [b, c, d] = obj;
 // Returning 3
 // Already reached the end (the last call returned `done: true`),
 // so `return` is not called
+console.log([b, c, d]); // [1, 2, undefined]; the value associated with `done: true` is not reachable
 
 for (const b of obj) {
   break;