documentation: clarify how the child actor get its name

[rethab]

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Helpful things

Fixes

Fixes #xxxx

Purpose

What does this PR do?

Background Context

It adds a tiny bit to the akka documentation clarifying how the child actor gets its name, because in the existing example the key parameter is used both as a parameter to the factory as well as the actor's name.

References

Are there any relevant issues / PRs / mailing lists discussions?

• this confused me a lot when I saw that the key is passed twice.