Call out type extensions not being allowed on type abbreviations more prominently

[cartermp]

From #1178. Currently, this tidbit of info is nestled away in a paragraph. It should be more prominent.

[BillWagner]

This issue has been closed as part of the issue backlog grooming process outlined in #22351.

That automated process may have closed some issues that should be addressed. If you think this is one of them, reopen it with a comment explaining why. Tag the @dotnet/docs team for visibility.