See symfony/symfony#25892

Also, our reference.rst section about the different cache adapters isn't great :).

The cache adapter used by the cache.app service. The FrameworkBundle
ships with multiple adapters: apcu, doctrine, system, filesystem,
psr6, redis and memcached.

It's not clear what values you would actually use, like cache.adapter.redis. It's also not very clear how you would configure, for example, the memcached connection details.