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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master

Details
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https://git.postgresql.org/pg/commitdiff/d5f788b41dc2cbdde6e7694c70dda54d829a5ed5

Modified Files
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src/backend/executor/nodeModifyTable.c | 70 ++++++++++++++++++++--------------
1 file changed, 41 insertions(+), 29 deletions(-)