Clarify gitlab:gitaly:check needs run from Gitaly client

What does this MR do?

Clarifies that sudo gitlab-rake gitlab:gitaly:check should be run from the rails node (or other Gitaly client).

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
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Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
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• Add the product tier badge accordingly.
• Add GitLab's version history note(s).
• Add/update the feature flag section.

Review checklist

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1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

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  • Add twdoing when starting work on the MR.
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