Update documentation.md

Author: Hakorr

documentation.md

@@ -22,6 +22,8 @@
 Grant the userscript `GM.xmlHttpRequest` and require the UserGui, like so.
 
 ```js
+// @grant       GM_getValue
+// @grant       GM_setValue
 // @grant       GM.xmlHttpRequest
 // @require     https://raw.githubusercontent.com/AugmentedWeb/UserGui/main/usergui.js
 ```