this should be an attribute, not an element, to match our XML conventions. The otherxsd:string elements that cannot occur multiple times should have been attributes too, and we already added them as attributes when documenting the XML config and finding this out. So I suggest that new settings are added only as attribute to avoid continuing the non-conventional format.

And what is the difference betweendefault-doctrine-dbal-provider anddefault-doctrine-provider ?

I favor consistency on this one.

And what is the difference between default-doctrine-dbal-provider and default-doctrine-provider ?

default-doctrine-provider should have been nameddefault-doctrine-cache-provider, it's using an adapter to doctrine/cache