improve linking between proxy config sections

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 
Not using ``framework.trusted_proxies`` as I first proposed [here](https://github.com/symfony/symfony-docs/pull/4102/files#r16030967) makes sense since the option name is already mentioned before in this sentence.