open.2, prctl.2: SYNOPSIS: Document these as variadic functions

open.2, prctl.2: SYNOPSIS: Document these as variadic functions

I remember I discussed this with Michael Kerrisk a long ago and we
agreed to apply this fix, as I felt that using overload syntax was
confusing (especially since C doesn't allow overloads), but then I
didn't feel urged to write a patch.  Florian confirmed recently that
this is confusing to more programmers, so let's do it.

Link: <https://bugzilla.kernel.org/show_bug.cgi?id=216876#c1>
Reported-by: Florian Weimer <fweimer@redhat.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>