Update documentation using "framework.cache.pools"

[tina-junold]

Since Symfon 3.1 for serializer and 3.2 for validation the deprecation handler warns about to use "framework.cache.pools" for both components instead of "framework.serializer.cache" and "framework.validation.cache". I've not able to fix the configuration by my self (maybe due to a lack of configuration documentation for symfony/cache).