Clarify instructions for updating PR checks to avoid emphasis on main

[mbg]

Since changes are typically introduced by PRs and all required PR checks need to pass before a PR can be merged, the update-required-checks.sh script likely needs to be run from the HEAD of a PR branch if the PR removes any checks or changes their names. This PR updates the relevant documentation so that there's less emphasis on running the script against main.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull Request Overview

This PR improves the documentation for the update-required-checks.sh script to clarify when the script should be run against a PR branch rather than the main branch. The changes reduce emphasis on running the script against main and provide clearer guidance for scenarios where PR branches should be used instead.

Key changes:

• Clarifies that the script should be run from a PR branch when that PR adds/removes checks
• Restructures documentation to better explain the script's flexibility with commit SHA arguments
• Removes outdated comment that emphasized main as the typical usage

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
CONTRIBUTING.md	Updated documentation to clarify script usage patterns and provide better guidance on when to use PR branches vs main
.github/workflows/script/update-required-checks.sh	Removed comment that emphasized main branch as typical usage

[henrymercer]

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