In section 2.6. The Default Authentication Manager -
We've touched on the idea that the namespace configuration automatically registers an authentication manager bean for you. This is an instance of Spring Security's ProviderManager class, which you may already be familiar with if you've used the framework before. You can't use a custom AuthenticationProvider if you are using either HTTP or method security through the namespace, but this should not be a problem as you have full control over the AuthenticationProviders that are used.
The 3rd Sentence "You can't use a custom 'AuthenticationProvider' if you are using either ... " should it be 'AuthenticationManager instead?