Add custom System.Object serializer and persistence default serializer peculiarity documentation

[Arkatufus]

Persistence have its own default System.Object serializer settings here:

akka.persistence.journal-plugin-fallback.serializer = json
akka.persistence.snapshot-store-plugin-fallback.serializer = json

https://github.com/akkadotnet/akka.net/blob/3f7d3980fa45565abb824fe5f192f87c1bdde113/src/core/Akka.Persistence/persistence.conf#L119
https://github.com/akkadotnet/akka.net/blob/dev/src/core/Akka.Persistence/persistence.conf#L178

This is a big gotcha for people who does not know about this peculiarity and thinks that Akka persistence would use the custom System.Object serializer they configured with Akka, as shown in issue #127

This needs to be better documented

[Aaronontheweb]

I think we can close this now that akkadotnet/akka.net#6389 is resolved