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

Documentation/admin-guide/tainted-kernels.rst

@@ -182,3 +182,5 @@ More detailed explanation for tainting
      produce extremely unusual kernel structure layouts (even performance
      pathological ones), which is important to know when debugging. Set at
      build time.
+
+ 18) ``N`` if an in-kernel test, such as a KUnit test, has been run.