"Hidden" signals used for `replicatesignal`

[Stefanuk12]

Most of the signals you try to use with replicatesignal will error, saying it's not found in the class. For example, Player.ConnectDiedSignalBackend and Player.Kill. An extension needs to be made to these classes, so these errors don't show. However, it needs to be made very clear that they can only be used with replicatesignal.