Fix sync docs

[Khaledgarbaya]

The the sync api endpoint does not support include or link resolution, the idea of the sync api is to have the ability of keeping a destination in sync with a contentful space as such it only transfer delta changes
and doesn't care at all about relationships. However contentful.js is doing link resolution client side if you only make an initial sync, for the second sync it is not possible since the sdk wont have access to all the data to make such an operation.