Document that instance types must match when restoring a backup

What does this MR do?

This MR documents that GitLab backups can only be restored on instances that match the type of instance it was backed up from (CE or EE).

Are there points in the code the reviewer needs to double check?

Phrasing

Why was this MR needed?

It is not otherwise clear that the instance types must match. It is only specified that the versions must match.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?