Improve `Actor` documentation to mention looping

[llucax]

What's needed?

We need to make it more clear that normally Actors are supposed to be constantly running.

Proposed solution

Add more loops in run() methods in examples and mention the looking in the general docs describing what actors are.

[llucax]

Looping is actually mentioned quite explicitly, and all examples actually have an infinite loop in the _run(), so I'm not sure what I was looking at when creating this issue.