exitApp should be documented

A lot of use use the `window.navigator.app.exitApp` functionality.
I think the entire `window.navigator.app` API should be documented. This will make it clear if it's a public facing API or not.
This will also open up the possibility of adding in [typescript definitions](https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/cordova).

I can't find another better built-in replacement for this API. There is at least [one reference](https://cordova.apache.org/docs/en/11.x/reference/cordova-plugin-splashscreen/#:~:text=navigator.app.exitApp()) in the current documentation to this api. I think there were more references in previous versions of the Cordova documentation.

Start by reviewing the existing Cordova documentation, especially the splashscreen reference to navigator.app.exitApp(), and determine where the window.navigator.app API is documented. Inventory the API surface covered by the issue, clarify whether it is public, and consider the linked TypeScript definitions as context. Done means the relevant API is documented with its public status made clear.

Written by the indexing model from the issue text.