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_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/a113a66a7a7dc28d176663067b5edc9065de2c71

Modified Files
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doc/src/sgml/func.sgml | 73 +++++++++++++++++++++++++++++++++---------------
1 file changed, 50 insertions(+), 23 deletions(-)