pgsql: Improve documentation of random() function.

Improve documentation of random() function.

Move random() and setseed() to a separate table, to have them grouped
together. Also add a notice that random() is not cryptographically secure.

Back-patch of commit 75fdcec14543b60cc0c67483d8cc47d5c7adf1a8 into
all supported versions, per discussion of the need to document that
random() is just a wrapper around random(3).

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/bdde191c1c79d86d43939435bec6a86c15d21041

Modified Files
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doc/src/sgml/func.sgml |   77 +++++++++++++++++++++++++++++++-----------------
1 file changed, 50 insertions(+), 27 deletions(-)