Revise comments in iset.mm
Revise comments in iset.mm

This is to remove references to theorems and syntax not yet
defined, in the comments for df-fun , df-fn , df-f , df-f1 ,
df-fo , df-f1o , and df-fv . With a few exceptions ( dffun3
and dffun5 most notably ) we should be able to bring back
the old text later.

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