add doc for lnnvl feature - #300

Add Chinese and English documentation for the Oracle compatible LNNVL
function, following the two-document layout used for the other built-in
functions: an implementation note under oracle_builtin_functions and a
usage document under oracle_compatibility.

The implementation notes cover the equivalence with the SQL standard
IS NOT TRUE predicate, the SQL definition and the reason for each of its
properties, why CALLED ON NULL INPUT is mandatory and what breaks if it is
changed to STRICT, and function inlining.

The usage documents cover the truth table, the partition property that
distinguishes LNNVL from a plain NOT, the NOT IN over a NULL-containing
subquery case, argument type checking, and the behavioural differences from
Oracle, including that the accepted argument forms are a superset of
Oracle's.

Also fix a duplicated entry number in the English navigation, where
dbtimezone was numbered 24 like the preceding entry; it is now 25, matching
the Chinese navigation, and LNNVL follows as 26 in both.