Update backup_restore.md to specify which Fog storage drivers are imported by default.

What does this MR do?

Updates documentation to note that GitLab only includes Fog libraries for a subset of available storage drivers.

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

Documentation only; please check to make sure the libraries I've listed are correct. The applicable line in the Gemfile is linked.

Why was this MR needed?

The documentation is ambiguous as to whether all the Fog storage drivers are available; they aren't.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/20861

Does this MR meet the acceptance criteria?

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