clean up cache invalidation information on the cache chapter

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs? | no
| Applies to    | all
| Fixed tickets | -

The documentation on active cache invalidation is overly negative. I tried to make it more neutral and point to FOSHttpCacheBundle for further information.
you mean the note that a cache can be invalidated? i can see to make that clear again for people really not familiar with caches...