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Me respondo solo:

Código SQL [-]
CREATE OR REPLACE FUNCTION pruebas(varchar, varchar, integer)
RETURNS void 
AS 
$delimiter$
INSERT INTO documentos(descripcion) SELECT $1
WHERE NOT EXISTS ( SELECT id FROM documentos WHERE descripcion = $1 );
INSERT INTO personal ( nombre, idtipodocumento, numdoc )
SELECT $2, ( SELECT id FROM documentos WHERE descripcion = $1 ), $3;
$delimiter$
LANGUAGE SQL;