Update custom domain docs to mention limitations around additional DNS records.

[luca-aurelia]

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
• 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• Relates to an existing (potentially outdated) blog article?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.

[itowlson]

The CI failure is unrelated to this PR - #1114 refers

[macolso]

Left a comment with some inline changes (and optionally a suggestion about when the warning should be displayed, but feel free to take or leave it). Thanks again for the contribution, @photon-garden!

[macolso]

Thanks for making this edit! I made a few wording adjustments with this inline suggestion. One other consideration to take or leave: would it be possible to move this suggestion towards to the top of the article under a ## Limitations header so users are aware a bit earlier in the tutorial in case it is a blocker for them?

Suggested change

Be aware that updating your domain's nameserver records will cause any additional records you've set through you registrar to no longer take effect. This includes, for example, TXT or MX records for services like Google Workspace. You can work around this by connecting your Fermyon Cloud app to a subdomain. But if you need to use the apex domain (example.com rather than app.example.com) and you also need to set additional DNS records, you should choose a different hosting service for the time being.

>> Additional records for custom domains are not supported on Fermyon Cloud at this time. This means any additional records (such as TXT or MX) you've set through your registrar no longer take effect once you've delegated your domain. If you need to assign your Spin application to an apex domain (`example.com` rather than `app.example.com`) _and_ you also need to set additional DNS records associated with that domain, please share this [feedback here](https://github.com/fermyon/feedback/issues/47). In the meantime, please consider delegating a subdomain or using a Fermyon custom subdomain for your Spin application instead.

[tpmccallum]

Thanks so much for this PR @photon-garden
I see that Kenzie has also added some additional options. Thanks for bringing this to our attention. I will merge this now and consider taking your message/information to a higher level on the page's real estate (by following the suggestion to create a Limitations heading). This will give users the chance to understand the limitations before they dive in.

[tpmccallum]

Thanks again @photon-garden for flagging this.
Please see continuation of this PR at #1127