kthread: add kerneldoc for kthread_create()

This macro is referenced in other kerneldoc comments, but lacks one of its
own; fix that.

Link: http://lkml.kernel.org/r/20160826072313.726a3485@lwn.net


Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Reported-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>