Обсуждение: pgsql: doc: in contrib-spi, mention and link to the meaning of SPI

doc:  in contrib-spi, mention and link to the meaning of SPI

Also remove outdated comment about SPI subtransactions.

Reported-by: gregory@arenius.com

Discussion: https://postgr.es/m/151726276676.1240.10501743959198501067@wrigleys.postgresql.org

Backpatch-through: 9.3

Branch
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REL9_3_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/2d58ef3469e14d8b4f62c6bef98cb7c313ede2cc

Modified Files
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doc/src/sgml/contrib-spi.sgml | 4 +++-
doc/src/sgml/spi.sgml         | 3 +--
2 files changed, 4 insertions(+), 3 deletions(-)