Docs: Relationship between ActorContext and Behavior in Typed

[otto-dev]

I think the following explanation (see link) by @johanandren regarding the relationship between Behaviors and ActorContexts would make a fine addition to the docs - if nowhere else, at least in the Learning Akka Typed from Classic section.

https://discuss.lightbend.com/t/in-akka-typed-what-constitutes-an-actor/5684/2

[mghildiy]

I can take this up.

[Captain1653]

This issue is quite old.

I can add this explanation in the docs if it's still needed. When should I add this? Create a new section before ActorContext.children?

[patriknw]

Seems like a useful clarification to add to the docs, please add it.