pgsql: Update comments for rewriteTargetListIU().

Update comments for rewriteTargetListIU().

This function's behavior for UPDATE on a trigger-updatable view was
justified by analogy to what preptlist.c used to do for UPDATE on
regular tables. Since preptlist.c hasn't done that since 86dc90056,
that argument is no longer sensible, let alone convincing. I think
we do still need it to act that way, so update the comment to explain
why.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/08a986966524e522914b96e4398a4bebf942b298

Modified Files
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src/backend/rewrite/rewriteHandler.c | 10 ++++------
1 file changed, 4 insertions(+), 6 deletions(-)