pgsql: Improve commentary about run-time partition pruning datastructu

Improve commentary about run-time partition pruning data structures.

No code changes except for a couple of new Asserts.

David Rowley and Tom Lane

Discussion: https://postgr.es/m/CAKJS1f-6GODRNgEtdPxCnAPme2h2hTztB6LmtfdmcYAAOE0kQg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4e23236403336052a000c8bf85720e9d65ba8036

Modified Files
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src/backend/optimizer/path/allpaths.c |  3 ++-
src/backend/partitioning/partprune.c  | 39 +++++++++++++++++++------------
src/include/executor/execPartition.h  | 43 +++++++++++++++++------------------
src/include/nodes/plannodes.h         | 27 ++++++++++++++++++----
src/include/nodes/relation.h          |  4 ++--
5 files changed, 71 insertions(+), 45 deletions(-)