docs(api-5): document wildcard parameters in req.params

Author: krzysdz

_includes/api/en/5x/req-params.md

@@ -1,13 +1,25 @@
 <h3 id='req.params'>req.params</h3>
 
-This property is an object containing properties mapped to the [named route "parameters"](/{{ page.lang }}/guide/routing.html#route-parameters). For example, if you have the route `/user/:name`, then the "name" property is available as `req.params.name`. This object defaults to `{}`.
+This property is an object containing properties mapped to the [named route "parameters"](/{{ page.lang }}/guide/routing.html#route-parameters). For example, if you have the route `/user/:name`, then the "name" property is available as `req.params.name`. This object defaults to `Object.create(null)`.
 
 ```js
 // GET /user/tj
 console.dir(req.params.name)
 // => "tj"
 ```
 
+Properties corresponding to wildcard parameters are arrays containing separate path segments split on `/`:
+
+```js
+app.get('/files/*file', (req, res) => {
+  console.dir(req.params.file)
+  // GET /files/note.txt
+  // => [ 'note.txt' ]
+  // GET /files/images/image.png
+  // => [ 'images', 'image.png' ]
+})
+```
+
 When you use a regular expression for the route definition, capture groups are provided in the array using `req.params[n]`, where `n` is the n<sup>th</sup> capture group.
 
 ```js