Add watchQuery: true (nuxt#796)

Source: https://github.com/nuxt/nuxt.js/blob/master/lib/app/client.js#L121

Author: macoshita

en/api/pages-watchquery.md

@@ -6,10 +6,12 @@ description: Watch query strings and execute component methods on change (asyncD
 # The watchQuery Property
 
 > Watch query strings and execute component methods on change (asyncData, fetch, validate, layout, ...)
-- **Type:** `Array` (default: `[]`)
+- **Type:** `Boolean` or `Array` (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.
 
+If you want to set up a watcher for all query strings, set `watchQuery: true`.
+
 ```js
 export default {
   watchQuery: ['page']