Update http_cache.rst

Explained how the parameters are passed to the controller - named parameters
Hmm, I agree we can highlight this better. But it's also true of `render`, not only `render_esi`. Also, I think the standards are `maxPerPage` and the controller name should be `contentAction` (since most people are probably using the normal shortcut syntax for the controller.

We *do* show the `$max` variable being passed as an argument in the next example. Perhaps we just need to add a note about that. For example:

```rst
.. tip::

    Any extra variables you pass (like ``max`` in the above example) will become arguments
    to your controller (e.g. ``$max``). You can see this in the controller below.
```

What do you think?