Currently, if a BeanDefinitionReader is registered via @ImportResource(reader = ...), the reader type must be public and have a public constructor that accepts a single BeanDefinitionRegistry argument.

In order to support registration of a custom, non-public BeanDefinitionReader, we should relax the undocumented visibility restriction.

In addition, we should document the fact that the BeanDefinitionReader's constructor must accept a single BeanDefinitionRegistry argument.