docs: clarify custom domain with user/org site

[jamietanna]

As a follow-up to support ticket 4079946 to make it more explicit.

Why:

Closes:

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

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• 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.

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

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

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[Copilot]

Pull request overview

This PR updates the GitHub Pages custom domain documentation to more explicitly state when a custom domain configured on a user/organization site applies across project sites, addressing a support-driven clarity gap.

Changes:

• Clarifies that the “default custom domain applies to all project sites” behavior only applies when the domain is set on a user or organization site.
• Keeps the existing reference link to the “types of sites” section for additional context.

[Copilot]

The added sentence "This will only work if the custom domain is set on a user or organization site." is redundant with the preceding clause (it restates the same condition) and can read as confusing/tautological. Consider rephrasing to clarify the contrast instead (e.g., that the default only applies when the custom domain is configured on the user/org site, not when it's configured on a project site).