Documentation: Add detailed explanation for 'N' taint flag

Every taint flag has an entry in the "More detailed explanation" section
except for the 'N' flag. That omission was probably just an oversight so
add an entry for that flag.

Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240717203521.514348-1-bpoirier@nvidia.com