Updated the description of cache warmers
bug #3689 Fix cache warmer description (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
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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.

Commits
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ac0f66d Updated the description of cache warmers