Document that watchQuery can be a function

en/api/pages-watchquery.md

@@ -4,7 +4,7 @@ description: Watch query strings and execute component methods on change (asyncD
 ---
 
 > Watch query strings and execute component methods on change (asyncData, fetch, validate, layout, ...)
-- **Type:** `Boolean` or `Array` (default: `[]`)
+- **Type:** `Boolean` or `Array` or `Function` (default: `[]`)
 
 Use the `watchQuery` key to set up a watcher for query strings. If the defined strings change, all component methods (asyncData, fetch, validate, layout, ...) will be called. Watching is disabled by default to improve performance.
 
@@ -15,3 +15,15 @@ export default {
   watchQuery: ['page']
 }
 ```
+
+You can also use the function `watchQuery(newQuery, oldQuery)` to have more refined watchers.
+
+```js
+export default {
+  watchQuery (newQuery, oldQuery) {
+    // Only execute component methods if the old query string contained `bar`
+    // and the new query string contains `foo`
+    return newQuery.foo && oldQuery.bar
+  }
+}
+```