Explain locking solution of Aggregates and Sagas

[smcvb]

It would be beneficial if the Reference Guide explained how Axon Framework locks an Aggregate and a Saga.
Although these specifics can be retrieved from the JavaDoc, a short section for Aggregates and Sagas may prove useful for readers.

When picking up this issue, it is helpful to take this forum post into account.
The post contains an explanation of how it works. Secondly, it formed the basis for defining this issue.