Clarify when a connection pool is created for Redis

[snicoll]

While reviewing #26326, I've noticied the following sentence in the documentation:

By default, if commons-pool2 is on the classpath, you get a pooled connection factory.

That's actually wrong at the moment. You need both that and setting at least one property of the .pool namespace (which is a bit confusing). When merging #26326 we can put that sentence back but we should rephrase things for the time being.