Clarify Copilot rate limit troubleshooting guidance

[ded-furby]

Summary

• update the rate-limit troubleshooting text so it no longer assumes every client shows a retry timestamp
• document that the message may instead tell people to wait for reset or upgrade for additional usage
• keep the article linked to the usage-limits guidance for next steps

Closes #43998

Validation

• git diff --check
• content-only review of content/copilot/how-tos/troubleshoot-copilot/troubleshoot-common-issues.md
• I did not run the full docs content linter in this temporary sparse checkout

[github-actions]

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Source	Review	Production	What Changed
copilot/how-tos/troubleshoot-copilot/troubleshoot-common-issues.md	fpt ghec	fpt ghec

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

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[ahmedrocky55436-stack]

job-logs.txt