Merge pull request #3127 from pngwn/3029-module-reactivity

Site: document that module script variables are not reactive. Closes …

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

@@ -176,6 +176,8 @@ You can `export` bindings from this block, and they will become exports of the c
 
 You cannot `export default`, since the default export is the component itself.
 
+> Variables defined in `module` scripts are not reactive — reassigning them will not trigger a rerender even though the variable itself will update. For values shared between multiple components, consider using a [store](https://svelte.dev/docs#svelte_store).
+
 ```html
 <script context="module">
 	let totalComponents = 0;