added a note about php-cgi and Apache

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.1+)
| Fixed tickets | symfony/symfony#7170, symfony/symfony#7649

People get stuck with Apache under php-cgi. We have a workaround in Symfony (symfony/symfony#3551) but it was not documented.