chore: [release-2.9.x] docs: Clarify Unit Requirement for Duration Comparison in Label Values

[grafanabot]

Backport 5570855 from #13318

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What this PR does / why we need it:
This update clarifies the requirement for units within the original label value when making a Duration comparison. It ensures that users understand the necessity of including a unit suffix (e.g., 'ms', 'h') for accurate parsing and comparison.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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