Document customizing `express.json()` size limit for specific routes

In the following issue, it is requested to customize the body size limit for `express.json()` dynamically based on the API route:

> @marbuser (Apr 11, 2019):
> I've got some endpoints and 90% of them I want to have a limit of around '2MB'. However, there is 1 specific route, my upload route, where I want to have an upload limit of '10MB'.

- https://github.com/expressjs/express/issues/3932

There are some solutions provided in the thread, but maybe a simpler option would be this one inspired by [this Stack Overflow answer](https://stackoverflow.com/questions/74387929/increase-body-limit-for-specific-route-with-expressjs):

```js
// Parse application/json
// - limit on /upload: 5MB
// - limit on other routes: default of 100KB
app.use('/upload', express.json({ limit: '5mb' }));
app.use(/^(?!\/upload$)/, express.json());
```

This may be a nice example to document on [the `express.json()` docs section](https://expressjs.com/en/5x/api.html#express.json)