docs: make kernel-doc handle varargs properly

As far as I can tell, the handling of "..." arguments has never worked
right, so any documentation provided was ignored in favor of "variable
arguments."  This makes kernel-doc handle "@...:" as documented.  It does
*not* fix spots in kerneldoc comments that don't follow that convention,
but they are no more broken than before.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>