pgsql: Add isolation tests for UPDATE/DELETE FOR PORTION OF

Add isolation tests for UPDATE/DELETE FOR PORTION OF

Add documentation about concurrency issues related to UPDATE/DELETE
FOR PORTION OF as well as supporting isolation tests.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Reviewed-by: Peter Eisentraut <peter@eisentraut.org>
Discussion: /messages/by-id/ec498c3d-5f2b-48ec-b989-5561c8aa2024@illuminatedcomputing.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b6ccd30d8ff6422ad0f79ce2fc801f2437d90664

Modified Files
--------------
doc/src/sgml/dml.sgml | 38 +
doc/src/sgml/images/Makefile | 3 +-
doc/src/sgml/images/meson.build | 1 +
doc/src/sgml/images/temporal-isolation.svg | 47 +
doc/src/sgml/images/temporal-isolation.txt | 44 +
src/backend/executor/nodeModifyTable.c | 4 +
src/test/isolation/expected/for-portion-of.out | 4089 ++++++++++++++++++++++++
src/test/isolation/isolation_schedule | 1 +
src/test/isolation/specs/for-portion-of.spec | 597 ++++
9 files changed, 4823 insertions(+), 1 deletion(-)