pgsql: doc: mention trigger helper functions in CREATE TRIGGER docs

doc: mention trigger helper functions in CREATE TRIGGER docs

Reported-by: petermpallesen@gmail.com

Discussion: /messages/by-id/159195294959.673.5752624528747900508@wrigleys.postgresql.org

Backpatch-through: 9.5

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/dd2bb77c72f4af9b6e546d02e35e84668a909950

Modified Files
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doc/src/sgml/ref/create_trigger.sgml | 6 ++++++
1 file changed, 6 insertions(+)