Hint at the reactivity limitation of array methods in the api docs (#2622)

* Hint at the reactivity limitation of array methods in the api docs

* tweaks

site/content/docs/01-component-format.md

@@ -56,6 +56,8 @@ To change component state and trigger a re-render, just assign to a locally decl
 
 Update expressions (`count += 1`) and property assignments (`obj.x = y`) have the same effect.
 
+Because Svelte's reactivity is based on assignments, using array methods like `.push()` and `.splice()` won't automatically trigger updates. Options for getting around this can be found in the [tutorial](tutorial/updating-arrays-and-objects).
+
 ```html
 <script>
 	let count = 0;