Update detaching-a-fork.md

[jkopczyn]

Clarify what to do with an error message about refusing to delete branches.

Why:

Got a somewhat scary message when the repository was old enough to use master and the new copy used main. Add a warning about this for future reference.

What's being changed (if available, include any code snippets, screenshots, or gifs):

Deleting a fork instructions; add a warning.

Check off the following:

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline (this link will be available after opening the PR).

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[github-actions]

Automatically generated comment ℹ️

This comment is automatically generated and will be overwritten every time changes are committed to this branch.

The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
pull-requests/collaborating-with-pull-requests/working-with-forks/detaching-a-fork.md	fpt ghec ghes@ 3.13 3.12 3.11 3.10 3.9	fpt ghec ghes@ 3.13 3.12 3.11 3.10 3.9

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

[nguyenalex836]

@jkopczyn Thanks so much for opening a PR! I'll get this triaged for review ✨

[jkopczyn]

I'm unclear on the linter's objection so I'll leave this for someone more familiar with the style guidelines to fix.

[nguyenalex836]

@jkopczyn Let me see if just updating the branch resolves 🤞 if not, one of our other team members can assist with the linter error upon reviewing this contribution 💛

[nguyenalex836]

@jkopczyn Thank you for your patience while the team reviewed!

After review, we believe the issue may be caused by initializing the new repository (creating a README) rather than an empty one.

If you can reach out to wonderful our support team to confirm this is correct and also a common problem, please let us know and we can work on making the "Create a repo" step clearer! ✨

For now, I'll close this issue - but feel free to reopen this whenever you have an update 💛 Thank you!