ownCloud 8: Added doc for addTranslations()

Adds documentation for the new mechanism for loading translations from https://github.com/owncloud/core/pull/11638

Please review @DeepDiver1975 @LukasReschke @butonic 
No, we want to prevent ajax **sync** loading, not **async** loading. Since not all apps will support **async** ajax loading of translations, there's a mechanism to pre-include translations using `OCP\Util\addTranslations('appName');` so they are directly available.

If apps still wish to use async loading, they need to explicitly call the async `OC.addTranslations()` function before using translations (or at app load time). Since that one is async, need to either pass a callback or act on the returned promise.