Discourage dvelimALT in iset.mm
Discourage dvelimALT in iset.mm

I tried to figure out what happened to dvelim* theorems in
more recent versions of set.mm and didn't quite get to the
bottom of it. So just add the discouraged tag for now, along
with a note about the naming.

metamath/set.mm,metamath/set.mm,metamath/set.mm