Clarify CNAME record value for private Pages sites in the subdomain section

[7kaji]

Why:

Closes:

There is no existing issue for this. It is a one-paragraph follow-up to a change the maintainers made themselves, so I opened a pull request directly. Happy to file an issue first if you would prefer that.

The "Configuring an apex domain and the www subdomain variant" section says that the CNAME record values are the same for publicly and privately published sites. That sentence was added in commit c76a1f56. The "Configuring a subdomain" section has no equivalent statement, even though that is where Changing the visibility of your GitHub Pages site sends the reader right after showing them the site's unique *.pages.github.io subdomain.

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

This adds one paragraph to "Configuring a subdomain", directly after the CNAME step. The first and third sentences reuse the wording of the apex and www section. The second sentence is new: it names the mistake the reader is most likely to make, because the unique subdomain is the only Pages URL shown in the repository settings for a privately published site.

Written as a paragraph rather than an alert: the section already has three alerts, the next line is a warning, and the change linked above used a paragraph too.

The link uses /enterprise-cloud@latest/ because the target article is GitHub Enterprise Cloud only, while this one is versioned for both Free, Pro & Team and Enterprise Cloud.

SME review requested for the second sentence. It follows from the existing instruction that "The CNAME record should always point to <user>.github.io or <organization>.github.io", and cname_to_github_user_domain? in github/pages-health-check does not match *.pages.github.io, but I have not found it stated by GitHub directly.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Source	Review	Production	What Changed
pages/configuring-a-custom-domain-for-your-github-pages-site/managing-a-custom-domain-for-your-github-pages-site.md	fpt ghec	fpt ghec

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

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