docs: Clarify Unit Requirement for Duration Comparison in Label Values

[nickgrafana]

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)
• [x] Documentation added
• [ ] Tests updated
• [ ] Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• [ ] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• [ ] For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• [ ] If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

All committers have signed the CLA.

[grafanabot]

This PR must be merged before a backport PR will be created.

[grafanabot]

This PR must be merged before a backport PR will be created.