Warn about NativeSessionStorage altering session.cache_limiter

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.1+
| Fixed tickets | (none)

We're implementing HttpFoundation (by itself) to replace our native session implementation and noticed that NativeSessionStorage [eliminates the global session.cache_limiter value](https://github.com/symfony/symfony/commit/39526df67c4bd8afe199a301734e556c8aa26b6b). I understand *why* it must do this, but users should be warned that the default usage prescribed in the manual will cause all pages to be sent with no cache-busting headers.

/cc @drak