docs: reload grafana with prometheus when enabling agent mode

[doc-claw-bot]

Summary

• update the Prometheus agent mode step to reload both Prometheus and Grafana
• clarify that the configuration change applies to both components

Test Plan

• not applicable (docs-only change)

[ti-chi-bot]

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[gemini-code-assist]

Code Review

This pull request updates the TiDB maintenance documentation to clarify that reloading the configuration applies changes to both Prometheus and Grafana, updating the corresponding tiup cluster reload command to include both roles. There are no review comments, so no additional feedback is provided.

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