pgsql: Add some comments about division of labor between rewriter and p

Add some comments about division of labor between rewriter and planner.

The rationale for the way targetlist processing is done wasn't clearly
stated anywhere, and I for one had forgotten some of the details. Having
just painfully re-learned them, add some breadcrumbs for the next person.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/efe4c9d7049f0bf832b792bfad05c92aaf86aa3c

Modified Files
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src/backend/optimizer/prep/preptlist.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)

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