docs: Add notes about serveStatic option

Author: shakyShane

lib/default-config.js

@@ -90,7 +90,12 @@ module.exports = {
 
     /**
      * Add additional directories from which static
-     * files should be served.
+     * files should be served. Should only be used in `proxy` or `snippet`
+     * mode.
+     * @property serveStatic
+     * @type Array
+     * @default []
+     * @since 2.8.0
      */
     serveStatic: [],