[Components][HttpKernel] outline implications of the kernel.terminate event

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4198
Yes, I agree with @cordoval - I think all we need to say is that you won't get the nice benefits unless you use PHP FPM. We might as well also mention that this is because the `fastcgi_finish_request()` PHP function is used, and only PHP FPM supports this.