kernel-doc: handle varargs cleanly

The method for listing varargs in kernel-doc notation is:
 * @...: these arguments are printed by the @fmt argument

but scripts/kernel-doc is confused:  it always lists varargs as:
	...	variable arguments
and ignores the @...: line's description, but then prints that
line after the list of function parameters as though it's
not part of the function parameters.

This patch makes kernel-doc print the supplied @...  description if it is
present; otherwise a boilerplate "variable arguments" is printed.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>