Reword the introduction and example for application config

[skirtles-code]

The original motivation here was to change the example. It showed app.config = {...}, which isn't realistic because you can't assign a new object to that property.

I originally considered giving an example of assigning an individual property but couldn't come up with a tidy example. Instead I changed it to log app.config, which shows how to access the object but not much else. In that context it no longer made sense to have the app.mount line, so I removed that. The sentence that preceded the example then seemed better placed after the example. Finally, I reworded the opening sentence.