Improve comments about why SET DEFAULT triggers must recheck for matches.

Improve comments about why SET DEFAULT triggers must recheck for matches.

I was confused about this, so try to make it clearer for the next person.

(This seems like a fairly inefficient way of dealing with a corner case,
but I don't have a better idea offhand.  Maybe if there were a way to turn
off the RI_FKey_keyequal_upd_fk event filter temporarily?)