Refine section on fine-tuning repository interfaces in the reference documentation

The section Fine-tuning Repository Definition of our reference docs should be clarified on why one would rather design their application repositories starting with Repository and not use a more advanced base repository. It makes sense to elaborate on the following concerns:

• Controlled method exposure – architects might want to avoid exposing methods like findAll() as they're potentially expensive operations on the underlying data store and should not be used unless really necessary (for example, in test cases).
• Defining other return types for collection methods – the core repository base interfaces declared Iterable as return type for CRUD collection methods. In 3.0 we have introduced additional interfaces that re-declare those methods using List as return type, but crafting a custom repository base interface can achieve that as well.