Add comment about omitting tail of backup filename on restore

What does this MR do?

This MR adds a comment to the Omnibus restore command, to clarify that tail end of the specified backup file that is being restored needs to be omitted. This is already mentioned in the Docker restore section, but should also be visible in the Omnibus restoration section.

Related issues

• A Premium customer encountered issues when trying to perform a restoration on an GitLab Omnibus installation, where specifying the full file name in the restoration command results in an error such as:

  The backup file 1683927138_2023_05_12_15.11.3-ee_gitlab_backup.tar_gitlab_backup.tar does not exist!

  This will occur if the full filename is specified instead of just 1683927138_2023_05_12_15.11.3-ee in this example.

  • Zendesk internal use only
  • Salesforce internal use only

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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  • Style Guide.
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If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
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    • Any task steps should be written as a numbered list.
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• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.