Updating limitations of custom domains

[tpmccallum]

Content must go through a pre-merge checklist.

Pre-Merge Content Checklist

This documentation has been checked to ensure that:

• The title, template, and date` are all set
• Does this PR have a new menu item (anywhere in templates/*.hbs files) that points to a document .md that is set to publish in the future? If so please only publish the .md and .hbs changes in real-time (otherwise there will be a menu item pointing to a .md file that does not exist)
• File does not use CRLF, but uses plain LF (hint: use cat -ve <filename> | grep '^M' | wc -l and expect 0 as a result)
• Has passed bart check

$ bart check content/cloud/faq.md
✅ content/cloud/faq.md
$ bart check content/cloud/custom-domain.md
✅ content/cloud/custom-domain.md
$ bart check content/cloud/custom-domains-tutorial.md
✅ content/cloud/custom-domains-tutorial.md

• Has been manually tested by running in Spin/Bartholomew (hint: use PREVIEW_MODE=1 and run npm run styles to update styling)
• Headings are using Title Case
• Code blocks have the programming language set to properly highlight syntax and the proper copy directive
• Have tested with npm run test and resolved all errors
• Relates to an existing (potentially outdated) blog article? If so please add URL in blog to point to this content.