Update modern-root-site.md

[calebmil]

Added warning to run Page Diagnostics before attempting root site swap. Aligns doc with the documentation for the same function in PowerShell.

[learn-build-service-prod[bot]]

Learn Build status updates of commit 4384664:

:bulb: Validation status: suggestions

File	Status	Preview URL	Details
SharePoint/SharePointOnline/modern-root-site.md	:bulb:Suggestion		Details

SharePoint/SharePointOnline/modern-root-site.md

• Line 100, Column 75: [Suggestion: preserve-view-not-set - See documentation] You've pinned this link to a specific version of content with the view parameter. It's recommended not to pin a version unless that version is A) not the default view and B) the context is about that version specifically. To proceed with pinning a version add the &preserve-view=true to the URL. Otherwise, remove the view parameter. URL: /microsoft-365/enterprise/page-diagnostics-for-spo?view=o365-worldwide

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[learn-build-service-prod[bot]]

Learn Build status updates of commit 6af9a58:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
SharePoint/SharePointOnline/modern-root-site.md	:white_check_mark:Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel