property primitives doco

[Kevin Ryde]

In the "Property Primitives" node of the manual (in the cvs), there
seems to be no mention of what parameters are passed to the
not_found_proc taken by primitive-make-property.

Experimenting shows it's the property and the object, but it'd be good
to say that, since I think one needs to know to write such a
procedure.

Also, the text for primitive-property-set! says "CODE", but the
prototype says "val".  Perhaps that's a bit of a typo, unless I
misunderstand it badly.