docs/frontend-system: bit more clear example of passing callbacks as output

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

docs/frontend-system/architecture/03-extensions.md

@@ -46,7 +46,9 @@ Each extension can define a configuration schema that describes the configuratio
 
 ### Factory
 
-The extension factory is the implementation of the extension itself. It is a function that is provided with any inputs and configuration that the extension received, and must produce the output that it defined. When an app instance starts up it will call the factory function of each extension that is part of the app, starting at leaf nodes and working its way up to the root of the app extension tree. The factory will only be called for active extensions, which is an extension that is not disabled and has an active parent. Extension factories should be lean and not do any heavy lifting or async work, as they are called during the initialization of the app, that should instead be deferred to the values shared through the extension outputs.
+The extension factory is the implementation of the extension itself. It is a function that is provided with any inputs and configuration that the extension received, and must produce the output that it defined. When an app instance starts up it will call the factory function of each extension that is part of the app, starting at leaf nodes and working its way up to the root of the app extension tree. The factory will only be called for active extensions, which is an extension that is not disabled and has an active parent.
+
+Extension factories should be lean and not do any heavy lifting or async work, as they are called during the initialization of the app. For example, if you need to do an expensive computation to generate your output, then prefer outputting a callback that does the computation instead. This allows the parent extension to defer the computation for later so that you avoid blocking the app startup.
 
 ## Creating an Extensions