workqueue: fix some docbook warnings

There are some errors in the docbook comments in workqueue.h that cause
warnings when the docs are built; this only recently came to light because
these comments were not used until now.  Fix the comments to make the
warnings go away.

The "args..." "fix" is a hack.  kerneldoc doesn't deal properly with named
variadic arguments in macros, so all I've really achieved here is to make
it shut up.  Fixing kerneldoc will have to wait for more time.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Tejun Heo <tj@kernel.org>