Clarify repository data moves for Gitaly Cluster

What does this MR do?

The approaches to copy/move repository data using tar, rsync, etc. will not work for Gitaly Cluster targets/destinations because they only go to a single host and because they bypass Praefect.

Data cannot be copied onto a single or all the nodes manually and be expected to work (or at least the behavior of doing this remains unclear from the perspective of Praefect lacking knowledge of what occurred behind it).

This change attempts to make more explicit on what approaches would work for Gitaly Cluster (only backup and restore) and what will not.

Related issues

Customer ticket: https://gitlab.zendesk.com/agent/tickets/259094

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