The `@rpc` annotation was registered with "unreliable" as the default
transfer_mode, but the runtime (SceneRPCInterface) defaults to
TRANSFER_MODE_RELIABLE. This caused a mismatch between the documented
default and the actual behavior when using Node.rpc_config().
Changed the parser default to "reliable" to match the runtime behavior
and C# RpcAttribute. Added comments in all three locations to help
prevent future drift.
Fixesgodotengine/godot-docs#8874
Related docs fix: godotengine/godot-docs#11554