Docs: Notifications: Add settings related to fallback mode
Docs: Notifications: Add settings related to fallback mode

and fixed remarks made on the WIP PR

vector-im/riot-android,vector-im/riot-android,vector-im/vector-android,vector-im/vector-android,vector-im/vector-android,vector-im/riot-android,vector-im/vector-android,vector-im/riot-android,vector-im/riot-android