Fix cache warmer description

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | -

- The warmers are also executed on the first request: https://github.com/symfony/HttpKernel/blob/master/Kernel.php#L568-571
- Defining optional warmers has some effects, saying it has no effect is wrong.