pgsql: Update some comments to clarify who does what in targetlist crea

Update some comments to clarify who does what in targetlist creation.

No code changes; just avoid blaming query_planner for things it doesn't
really do.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/96f990e23bae0559ede95e94340d2fc918dfde44

Modified Files
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src/backend/optimizer/plan/planner.c | 30 +++++++++++++++++-------------
1 files changed, 17 insertions(+), 13 deletions(-)