pgsql: Expand comments and add an assertion in nodeModifyTable.c.

Expand comments and add an assertion in nodeModifyTable.c.

Most comments concern RELKIND_VIEW.  One addresses the ExecUpdate()
"tupleid" parameter.  A later commit will rely on these facts, but they
hold already.  Back-patch to v12 (all supported versions), the plan for
that commit.

Reviewed (in an earlier version) by Robert Haas.

Discussion: https://postgr.es/m/20240512232923.aa.nmisch@google.com

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ad4653268c12cd50339dae0de59d9add41138d66

Modified Files
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src/backend/executor/nodeModifyTable.c | 24 +++++++++++++++++-------
1 file changed, 17 insertions(+), 7 deletions(-)