The GSG has a note warning that use of UIO is inherently unsafe due to
lack of IOMMU protection. However, this was only flagged as a "NOTE",
meaning it could easily be missed. Changing the rst tag from "note" to
"warning" and moving it to the top of the UIO subsection makes this a
lot more visible to users.
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>