`SceneMultiplayer.allow_object_decoding()` should specify that this does not work with custom objects

**Your Godot version: 4.6**

**Issue description:**
`SceneMultiplayer.allow_object_decoding` should specify that this **does not work** with custom objects.

For example:
```gdscript
# player.gd
class_name Player extends RefCounted

var id: int
var slot: int
var name: String

# lobby.gd
func connected_to_server() -> void:
# add data to player...
_register_player.rpc(player)

@rpc("any_peer", "reliable")
func _register_player(_player: Player) -> void:
# _player will be empty
```

**URL to the documentation page:**
https://docs.godotengine.org/en/stable/classes/class_scenemultiplayer.html#class-scenemultiplayer-property-allow-object-decoding