Clarify cross-referencing when creating an issue from a discussion

[Aimaghsoodi]

Summary

• clarify that using Create issue from discussion also creates a cross-reference between the discussion and the issue
• address the documentation gap described in creating-an-issue doesn't talking about cross-referencing / explain how to cross-reference #42376

Notes

• docs-only change
• opened as a draft to keep review/process lightweight
• happy to close this if I should route it differently

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Source	Review	Production	What Changed
issues/tracking-your-work-with-issues/using-issues/creating-an-issue.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14

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