lib: deprecate the default route and improve its documentation

docs/Reference/Server.md

@@ -1024,13 +1024,20 @@ const defaultRoute = fastify.getDefaultRoute()
 <a id="setDefaultRoute"></a>
 
 **Note**: The default 404 handler, or one set using `setNotFoundHandler`, will
-never trigger if the default route is overridden. This sets the handler for the 
+never trigger if the default route is overridden. This sets the handler for the
 Fastify application, not just the current instance context. Use
 [setNotFoundHandler](#setnotfoundhandler) if you want to customize 404 handling
-instead. Method to set the `defaultRoute` for the server:
+instead.
+
+This method set the `defaultRoute` for the server. Note that, its purpose is
+to interact with the underlying raw requests. Unlike other Fastify handlers, the
+arguments received are from type [RawRequest](./TypeScript.md#rawrequest) and
+[RawReply](./TypeScript.md#rawreply) respectively.
 
 ```js
 const defaultRoute = function (req, res) {
+  // req = RawRequest
+  // res = RawReply
   res.end('hello world')
 }