pgsql: doc: add documentation about ecpg Oracle-compatibility mode

doc: add documentation about ecpg Oracle-compatibility mode

Reported-by: Takeshi Ideriha

Discussion: /messages/by-id/TYCPR01MB7041A157067208327D8DAAF9EAA59@TYCPR01MB7041.jpnprd01.prod.outlook.com

Backpatch-through: 11

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/846ae360ab2d70f934af0603d5fec140059ed26f

Modified Files
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doc/src/sgml/ecpg.sgml | 39 ++++++++++++++++++++++++++++++++++++++-
1 file changed, 38 insertions(+), 1 deletion(-)