Add doc comment to FileList.item with iteration example

Author: nojaf

src/DOMAPI/FileList.res

@@ -2,6 +2,17 @@ open FileAPI
 open DOMAPI
 
 /**
+Returns the `File` at the specified index.
+
+`FileList` is not an array, so you need to iterate manually using `length` and `item`:
+
+```rescript
+let files = []
+for i in 0 to fileList.length - 1 {
+  files->Array.push(fileList->FileList.item(i))
+}
+```
+
 [Read more on MDN](https://developer.mozilla.org/docs/Web/API/FileList/item)
 */
 @send