Clarified the behaviour of sync in preview mode.
Clarified the behaviour of sync in preview mode.

akfreas/contentful.objc,contentful/contentful.objc,contentful/contentful.objc,contentful/contentful.objc,davidmdavis/contentful.objc,danieleggert/contentful.objc,akfreas/contentful.objc,danieleggert/contentful.objc,akfreas/contentful.objc,davidmdavis/contentful.objc,davidmdavis/contentful.objc,danieleggert/contentful.objc