[HttpFoundation] added a note about JSON responses as arrays (refs #6970)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6970
| License       | MIT
| Doc PR        | symfony/symfony-docs#2555