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 
@Raydiation you mean add back the sync loading of translations inthe JS `t()` as a fallback, at least to not break apps that forgot to add that line ? Might make sense, yes.