[WIP] Improve user context docs

The docs currently show the following confusing VCL snippet:

```
# A little hack for testing all scenarios. Choose one for your application.
        if ("failure" == req.http.x-cache-hash) {
            set req.url = "/user_context_hash_failure.php";
        } elsif (req.http.x-cache-hash) {
            set req.url = "/user_context_hash_cache.php";
        } else {
            set req.url = "/user_context_hash_nocache.php";
        }
```