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
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/832c095d686e89f6390e0ff3e91b4c8015c2c3f7

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