pgsql: Improve comments about why SET DEFAULT triggers must recheck for

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?)

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/48756be9cf31c37a936eecff051bf143d5866551

Modified Files
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src/backend/utils/adt/ri_triggers.c |   32 ++++++++++++++++++++------------
1 files changed, 20 insertions(+), 12 deletions(-)