docs: note array index access with this.[n] in #each

src/guide/builtin-helpers.md

@@ -108,6 +108,18 @@ The first and last steps of iteration are noted via the [`@first`](../api-refere
 Nested `each` blocks may access the iteration variables via depth based paths. To access the parent index, for example,
 `{{@../index}}` can be used.
 
+When the current item is itself an array, use
+[segment-literal notation](expressions.md#literal-segments) to read a fixed index. Plain `this[0]` is not valid
+Handlebars syntax; write `this.[0]` instead:
+
+```handlebars
+<ul class="tuples">
+  {{#each tuple}}
+    <li>{{this.[0]}} {{this.[1]}}</li>
+  {{/each}}
+</ul>
+```
+
 :::
 
 ## #with