GDScript: Fix incorrect default transfer_mode for @rpc annotation

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.

Fixes godotengine/godot-docs#8874
Related docs fix: godotengine/godot-docs#11554

modules/multiplayer/scene_rpc_interface.cpp

@@ -84,6 +84,7 @@ void SceneRPCInterface::_parse_rpc_config(const Variant &p_config, bool p_for_no
 		ERR_CONTINUE(config[name].get_type() != Variant::DICTIONARY);
 		ERR_CONTINUE(!config[name].operator Dictionary().has("rpc_mode"));
 		Dictionary dict = config[name];
+		// Default values should match GDScript `@rpc` annotation registration and `rpc_annotation()`.
 		RPCConfig cfg;
 		cfg.name = name;
 		cfg.rpc_mode = ((MultiplayerAPI::RPCMode)dict.get("rpc_mode", MultiplayerAPI::RPC_MODE_AUTHORITY).operator int());