br: clarify that the Azure AD systemd step edits the service config, not TiKV's config file

[yahonda]

What is changed, added or deleted? (Required)

In "Method 3: Use Azure AD for backup and restore", the step that runs systemctl edit tikv-24000 was followed by "Edit the TiKV configuration file to configure the three environment variables", which is inaccurate: systemctl edit opens a systemd unit drop-in override, not TiKV's own configuration file (tikv.toml).

The same document's newer "Method 4: Use Azure managed identities" section already describes the identical systemctl edit tikv-24000 step correctly, as "Open the systemd service editor". This PR aligns the older section's wording with the already-correct newer section.

Source-code verification: TiUP's systemd unit template (embed/templates/systemd/system.service.tpl) shows the [Service] section only runs run_tikv.sh via ExecStart — the unit itself has no reference to tikv.toml. TiKV's actual config file path is passed as a --config flag inside run_tikv.sh, a separate file untouched by systemctl edit. This confirms systemctl edit tikv-24000 can only ever modify the systemd service configuration (e.g. add Environment= lines), never TiKV's own configuration file.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)

What is the related PR or file link(s)?

• This PR is translated from:
• Other reference link(s):

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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• br/backup-and-restore-storages.md

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📝 Walkthrough

Walkthrough

The Azure Blob Storage authentication documentation now directs TiKV users to configure Azure AD environment variables through the systemd service configuration.

Changes

Azure authentication documentation

Layer / File(s)	Summary
Update Azure AD configuration instruction br/backup-and-restore-storages.md	The instructions now use systemctl edit to configure AZURE_CLIENT_ID, AZURE_TENANT_ID, and AZURE_CLIENT_SECRET for TiKV.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~2 minutes

Change: Other

Suggested reviewers: qiancai

Merge Risk: ⚪ Minimal · up to c6bec

The revised instructions clarify the existing systemd configuration workflow and introduce no merge-blocking risk.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the documentation change: clarifying that the Azure AD systemd step edits the service configuration, not TiKV's configuration file.
Description check	✅ Passed	The description explains the change and its rationale, identifies the affected v8.5 version, includes verification details, and completes the relevant template sections. The omitted first-time contrib…

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