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.

doc/src/sgml/func.sgml

@@ -82,8 +82,9 @@
      <member><literal>NOT</></member>
     </simplelist>
-    <acronym>SQL</acronym> uses a three-valued Boolean logic where the null value represents
-    <quote>unknown</quote>.  Observe the following truth tables:
+    <acronym>SQL</acronym> uses a three-valued logic system with true,
+    false, and <literal>null</>, which represents <quote>unknown</quote>.
+    Observe the following truth tables:
     <informaltable>
      <tgroup cols="4">