Clarify which Containers can update the system container:

- One Container can be 'in control' of the system container.
- The Factory interface describes how this happens.

Explain Mementos:

- A Memento and associated system resources are sufficient to reconstruct
  a Container value for an existing system container.
- A Memento need not contain all the state of a container.

Make explicit that Id() is a generated value.

Docker-DCO-1.1-Signed-off-by: Steven Powell <spowell@gopivotal.com> (github: Zteve)