Comment responsibility groups

After writing usage examples of our ValidationCommandBus, we now can
clearly see two separate responsibility groups repeating in each of our
examples - command validation and handling.

Multiple responsibility groups usually hint towards SRP (Single
Responsibility Principle) violation. In our case - ValidationCommandBus
clearly knows not only how to create and use validator, but also how to
create and use command.

This issue is common when you use inheritance. Inheritance effectively
hides mixing of multiple concerns, hence the mantra "Composition over
inheritance".