Site: document that module script variables are not reactive. Closes #3029.

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.
+
 ```html
 <script context="module">
 	let totalComponents = 0;