MultiplayerSpawner does not warn to use unique names for child scenes.

**Your Godot version:**

4.3 but the docs should be updated for all.

**Issue description:**

When adding a child to be spawned across clients, the node must have a unique name to the rest of the children. If not: the child will added on the server, but not spawned across clients. This requirement for unique scene names is not mentioned in the documentation, and it is natural to assume that the child will be given a unique name and *then* replicated. I think adding a warning about this requirement (and outlining usage in the docs) would make the MultiplayerSpawner easier to use.

**URL to the documentation page (if already existing):**

https://docs.godotengine.org/en/stable/classes/class_multiplayerspawner.html