[concept] clarify the term "tombstone" in the context of cb mobile

We should clearly define what a Tombstone is . The first statement "simply marked as deleted" can be misleading. Users may update the document via the SDK to use _deleted is true and expect to see the same tombstoned behavior as would be the case when doc is deleted through SGW REST API.

Also need to clarify that under convergence, tombstoned docs do not preserve doc body. So users should be cautioned from trying to add additional data to deleted documents and expect it be available in synced clients.

(CC Adam Fraser)