pgsql: Update documentation to state there is three-value logic, not

Update documentation to state there is three-value logic, not
three-value boolean logic.

Backpatch to 9.0.X since we just got another bug report about this
today.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/f71b5c77fa0015492a26838eb489293effee1723

Modified Files
--------------
doc/src/sgml/func.sgml | 5 +++--
1 files changed, 3 insertions(+), 2 deletions(-)