Docs: update custom domains docs (#11734)

Closes #11371
ref #11242


https://docs--11734.org.readthedocs.build/en/11734/guides/custom-domains.html

docs/user/guides/custom-domains.rst

@@ -20,15 +20,18 @@ To setup your :doc:`custom domain </custom-domains>`, follow these steps:
 #. At the top of the next page you'll find the value of the DNS record that you need to point your domain to.
    For |org_brand| this is ``readthedocs.io``, and for :doc:`/commercial/index`
    the record is in the form of ``<hash>.domains.readthedocs.com``.
-
-
-**Once you have completed these steps and your new DNS entry has propagated (usually takes a few minutes), you need to build your project's published branches again so the Canonical URL is correct.**
+   If you are using Cloudflare, make sure to disable the proxy status (orange cloud) for the CNAME record.
 
  .. note::
 
     For a subdomain like ``docs.example.com`` add a CNAME record,
     and for a root domain like ``example.com`` use an ANAME or ALIAS record.
 
+.. warning::
+
+   If you delete a domain, make sure to also remove the DNS records for that domain.
+   Otherwise, another user may add the same domain to their project and serve that content from your domain (domain hijacking).
+
 We provide a validated SSL certificate for the domain,
 managed by `Cloudflare <https://www.cloudflare.com/>`_.
 The SSL certificate issuance should happen within a few minutes,
@@ -58,13 +61,19 @@ To remove a custom domain:
 #. Click on :guilabel:`Domains`.
 #. Click the :guilabel:`Remove` button next to the domain.
 #. Click :guilabel:`Confirm` on the confirmation page.
+#. Remove the DNS record for the domain from your DNS provider.
 
 .. warning::
 
     Once a domain is removed,
     your previous documentation domain is no longer served by Read the Docs,
     and any request for it will return a 404 Not Found!
 
+.. warning::
+
+   If you delete a domain, make sure to also remove the DNS records for that domain.
+   Otherwise, another user may add the same domain to their project and serve that content from your domain (domain hijacking).
+
 Strict Transport Security (HSTS) and other custom headers
 ---------------------------------------------------------
 
@@ -97,6 +106,24 @@ This is `a way to boost the SEO of your website <https://moz.com/blog/subdomains
 Troubleshooting
 ---------------
 
+"Error 1014: CNAME Cross-User Banned" when using Cloudflare
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Read the Docs uses Cloudflare to manage SSL certificates for custom domains,
+CDN caching, and other features that require the domain to be completely managed by our Cloudflare account.
+
+If you see an "Error 1014: CNAME Cross-User Banned" message,
+it means that the domain is already managed by another Cloudflare account.
+To fix this, you need to:
+
+#. Log in your Cloudflare account (https://www.cloudflare.com/).
+#. Select your domain.
+#. Click on "DNS".
+#. Find your CNAME record and click on "Edit".
+#. Uncheck the "Proxy status" (orange cloud) option.
+#. Leave everything else unchanged.
+#. Click on save.
+
 SSL certificate issue delays
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~