Documentation : [Geo] Be specific about current limitations of Geo in documentation

I think https://docs.gitlab.com/ee/administration/geo/replication/index.html#current-limitations needs to be updated in the first place. I think we need a comprehensive list of items that won't be synced. It sounds weird that we list "examples of not replicated items", if we don't know the exact list, our customers don't know either. It sounds a little awkward IMHO

It's harder to maintain the exact list but I think is absolutely needed.