docs: clarify custom domain with user/org site

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

Author: jamietanna

content/pages/configuring-a-custom-domain-for-your-github-pages-site/about-custom-domains-and-github-pages.md

@@ -34,7 +34,7 @@ We recommend always using a `www` subdomain, even if you also use an apex domain
 
 ## Using a custom domain across multiple repositories
 
-If you set a custom domain for a user or organization site, by default, the same custom domain will be used for all project sites owned by the same account. For more information about site types, see [AUTOTITLE](/pages/getting-started-with-github-pages/what-is-github-pages#types-of-github-pages-sites).
+If you set a custom domain for a user or organization site, by default, the same custom domain will be used for all project sites owned by the same account. This will only work if the custom domain is set on a user or organization site. For more information about site types, see [AUTOTITLE](/pages/getting-started-with-github-pages/what-is-github-pages#types-of-github-pages-sites).
 
 For example, if the custom domain for your user site is `www.octocat.com`, and you have a project site with no custom domain configured that is published from a repository called `octo-project`, the {% data variables.product.prodname_pages %} site for that repository will be available at `www.octocat.com/octo-project`.