exitApp should be documented

[adipascu]

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.

I can't find another better built-in replacement for this API. There is at least one reference in the current documentation to this api. I think there were more references in previous versions of the Cordova documentation.

[breautek]

Moving this to the cordova docs repo, pull requests are welcome.