Mention browser APIs in Iterator (mdn#35598)

* Updating Iterator page

Some information about iterators is incomplete.

* Update Iterator

* Update index.md

* Update index.md

* Update index.md

* Update index.md

* Update index.md

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

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

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

files/en-us/web/javascript/reference/global_objects/iterator/index.md

@@ -22,6 +22,14 @@ The following are all built-in JavaScript iterators:
 - The _Segments Iterator_ returned by the [`[Symbol.iterator]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Segmenter/segment/Segments/Symbol.iterator) method of the [`Segments`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Segmenter/segment/Segments) object returned by [`Intl.Segmenter.prototype.segment()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/Segmenter/segment).
 - The _Iterator Helper_ returned by iterator helper methods such as {{jsxref("Iterator.prototype.filter()")}} and {{jsxref("Iterator.prototype.map()")}}.
 
+Web APIs may return iterators too. Some reuse core JavaScript iterators while others define their own iterators. For example:
+
+- [Array-Like](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array#array-like_objects) objects such as [`NodeList`](/en-US/docs/Web/API/NodeList) return an _Array Iterator_ from their respective methods `keys()`, `values()`, `entries()`, and `[Symbol.iterator]()`.
+- [Map-Like](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map#map-like_browser_apis) objects from Web APIs such as [`Headers`](/en-US/docs/Web/API/Headers) return their own iterator type like _Headers Iterator_ from their respective methods `keys()`, `values()`, `entries()`, and `[Symbol.iterator]()`.
+- [Set-Like](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set#set-like_browser_apis) objects from Web APIs such as [`FontFaceSet`](/en-US/docs/Web/API/FontFaceSet) return their own iterator type like _FontFaceSet Iterator_ from their respective methods `keys()`, `values()`, `entries()`, and `[Symbol.iterator]()`.
+
+> **Note:** [`NodeIterator`](/en-US/docs/Web/API/NodeIterator) and other old interfaces are named as such but do not conform to the [iterator protocol](/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#the_iterator_protocol) or [iterable protocol](/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#the_iterable_protocol).
+
 Each of these iterators have a distinct prototype object, which defines the `next()` method used by the particular iterator. For example, all string iterator objects inherit from a hidden object `StringIteratorPrototype`, which has a `next()` method that iterates this string by code points. `StringIteratorPrototype` also has a [`[Symbol.toStringTag]`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/toStringTag) property whose initial value is the string `"String Iterator"`. This property is used in {{jsxref("Object.prototype.toString()")}}. Similarly, other iterator prototypes also have their own `[Symbol.toStringTag]` values, which are the same as the names given above.
 
 All of these prototype objects inherit from `Iterator.prototype`, which provides a [`[Symbol.iterator]()`](/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/iterator) method that returns the iterator object itself, making the iterator also [iterable](/en-US/docs/Web/JavaScript/Reference/Iteration_protocols#the_iterable_protocol).