Pantheon Global CDN Doc Update

Re: [Pantheon Global CDN](https://docs.pantheon.io//guides/global-cdn/global-cdn-beta) Priority: Low/Medium/High (choose one, remove the other options) High? ## Issue Description: This section : https://docs.pantheon.io/guides/global-cdn/global-cdn-beta#will-my-site-have-downtime-during-migration States ``` Will my site have downtime during migration? No. The migration process is designed to avoid downtime. During DNS propagation, traffic may temporarily alternate between the old and new CDN, but your site remains accessible throughout. ``` This is false. Updating the DNS entties for a site thats been on pantheon for a few years has resulted in the site being down, and it reporting SSL errors on my computer, and in third party SSL utility tools <img width="1534" height="1030" alt="Image" src="https://github.com/user-attachments/assets/86761460-3d67-4736-98ce-d23aea1d84a3" /> ## Suggested Resolution The documentation should be updated with some details on expected downtime. Cause... I'm glad this is a slow / small site I'm using as a guinea pig... My larger clients would absolutely throw a fit.