Clarify that init callback can't be used from backend code and why (#5645)

docs/reference/src/language/builtins/callbacks.md

@@ -31,18 +31,33 @@ export component MyWindow inherits Window {
 ```
 
 Don't use this callback to initialize properties, because this violates the declarative principle.
-Avoid using this callback, unless you need it, for example, in order to notify some native code:
+
+Even though the `init` callback exists on all components, it cannot be set from application code,
+i.e. an `on_init` function does not exist in the generated code. This is because the callback is invoked during the creation of the component, before you could call `on_init` to actually set it.
+
+While the `init` callback can invoke other callbacks, e.g. one defined in a `global` section, and
+you _can_ bind these in the backend, this doesn't work for statically-created components, including
+the window itself, because you need an instance to set the globals binding. But it is possible
+to use this for dynamically created components (for example ones behind an `if`):
 
 ```slint,no-preview
-global SystemService  {
+export global SystemService  {
     // This callback can be implemented in native code using the Slint API
     callback ensure_service_running();
 }
 
-export component MySystemButton inherits Rectangle {
+component MySystemButton inherits Rectangle {
     init => {
         SystemService.ensure_service_running();
     }
     // ...
 }
+
+export component AppWindow inherits Window {
+    in property <bool> show-button: false;
+
+    // MySystemButton isn't initialized at first, only when show-button is set to true.
+    // At that point, its init callback will call ensure_service_running()
+    if show-button : MySystemButton {}
+}
 ```