Document global app properties

[chrismayer]

Since we are using global / app-wide properties, e.g. to access the application-config (Vue.prototype.$appConfig), we should think about documenting them. Other ones are $map, $isEmbedded, $appLanguage and cmpLookup. But this would imply to define which are defined for private use only and which ones would be considered as public API.

KUDOS to @sronveaux, who spotted this in #356.