pgsql: Doc: document the current-transaction-modes GUCs.

Doc: document the current-transaction-modes GUCs.

We had documentation of default_transaction_isolation et al,
but for some reason not of transaction_isolation et al.
AFAICS this is just an ancient oversight, so repair.

Per bug #17077 from Yanliang Lei.

Discussion: /messages/by-id/17077-ade8e166a01e1374@postgresql.org

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/35e9d26a1bfe0d4fb726edc5c03b8e4fcc0a9869

Modified Files
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doc/src/sgml/config.sgml | 62 +++++++++++++++++++++++++++++++++++
doc/src/sgml/ref/set_transaction.sgml | 13 +++++++-
2 files changed, 74 insertions(+), 1 deletion(-)