Update documentation on user rpc calls.

docs/backends.md

@@ -229,12 +229,31 @@ void MyBackend::my_rpc(const std::vector<std::uint8_t>& data)
 const rpc = MxRpc.Create();
 rpc.then((handle) => {
     handle.BckCallUserRpc([
-        MxGenericType.str32('MyBackend::rpc'),
+        MxGenericType.str32('my_backend'), // Executable name (as listed in home page)
         MxGenericType.int32(42, 'generic') // Custom data
     ]);
 });
 ```
 
+You can also bundle multiple data with the help of the `MxGenericType.makeData` and `MxGenericType.concatData` functions.
+
+```ts
+const rpc = MxRpc.Create();
+rpc.then((handle) => {
+
+    const data = MxGenericType.concatData([
+        MxGenericType.int8(42),
+        MxGenericType.f32(3.141592654)
+    ]);
+
+    handle.BckCallUserRpc([
+        MxGenericType.str32('my_backend'), // Executable name (as listed in home page)
+        MxGenericType.makeData(data, 'generic') // Custom data
+    ]);
+});
+```
+Of course proper data handling must take place at the backend user function.
+
 The plugin containing this code will call the backend user rpc function
 as long as it is connected to the mx system.