docs: Document req.params changes in 5.x

Fixes #2090

Author: krzysdz

en/guide/migrating-5.md

@@ -71,6 +71,7 @@ You can find the list of available codemods [here](https://github.com/expressjs/
   <li><a href="#app.router">app.router</a></li>
   <li><a href="#req.body">req.body</a></li>
   <li><a href="#req.host">req.host</a></li>
+  <li><a href="#req.params">req.params</a></li>
   <li><a href="#req.query">req.query</a></li>
   <li><a href="#res.clearCookie">res.clearCookie</a></li>
   <li><a href="#res.status">res.status</a></li>
@@ -506,14 +507,30 @@ const server = app.listen(8080, '0.0.0.0', (error) => {
 
 The `app.router` object, which was removed in Express 4, has made a comeback in Express 5. In the new version, this object is a just a reference to the base Express router, unlike in Express 3, where an app had to explicitly load it.
 
-<h3 id="req.body">req.body</h3> 
+<h3 id="req.body">req.body</h3>
 
 The `req.body` property returns `undefined` when the body has not been parsed. In Express 4, it returns `{}` by default.
 
 <h3 id="req.host">req.host</h3>
 
 In Express 4, the `req.host` function incorrectly stripped off the port number if it was present. In Express 5, the port number is maintained.
 
+<h3 id="req.params">req.params</h3>
+
+Parameters specified as wildcards are represented as arrays of segments:
+
+```js
+app.get('/*splat', (req, res) => {
+  // GET /foo/bar
+  console.dir(req.params)
+  // => [Object: null prototype] { splat: [ 'foo', 'bar' ] }
+})
+```
+
+Optional parameters that are not matched do not have a key in `req.params`. In 4.x an unmatched wildcard would be an empty string and a `:` parameter made optional by `?` had a key with value `undefined`.
+
+The object has null prototype.
+
 <h3 id="req.query">req.query</h3>
 
 The `req.query` property is no longer a writable property and is instead a getter. The default query parser has been changed from "extended" to "simple".