Adjust hint for backup path volume mount point

What does this MR do?

Just a more precise hint as to where exactly a Docker volume would have to point in order for backups to persist.

As per the docs, the GitLab data directory at /var/opt/gitlab gets a Docker volume.

However, for backups to persist, it would only be required to have a mount at /var/opt/gitlab/backups (so just the default of gitlab-backup [create|restore]. In fact, we can mount both, see here and here. For some, this might be a desired setup. For example, I have /var/opt/gitlab in a named Docker volume on a local SSD, whereas /var/opt/gitlab/backups is, through Docker-Compose, mounted as an external NFS share. This affords us good separation of concerns and is very easily implemented.

Technically, you can omit backing /var/opt/gitlab with a volume and only have /var/opt/gitlab/backups as a volume (would still persist backups at least), but that makes little sense.

Related issues

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened