docs: add lnnvl function documentation

Add the Oracle-compatible lnnvl function to the built-in functions
table and its description section in both the English and Chinese docs.

Closes #301

Signed-off-by: jokerzsd <2701819133@qq.com>

Author: jokerzsd

CN/modules/ROOT/pages/7.8.adoc

@@ -75,6 +75,7 @@
 |compose
 |decompose
 |sys_context
+|lnnvl
 |====
 
 == 内置函数说明
@@ -1150,4 +1151,12 @@ ivorysql=# select sys_context('USERENV','DB_NAME');
  ivorysql
 (1 row)
 
+```
+=== `lnnvl` 函数
+功能：当条件为 FALSE 或 UNKNOWN（NULL）时返回 TRUE，条件为 TRUE 时返回 FALSE。通常用于 WHERE 子句中，以包含条件为假或为 NULL 的行。测试用例如下：
+
+查询值不等于 10 或值为 NULL 的行：
+
+```
+select * from t where lnnvl(a = 10);
 ```

EN/modules/ROOT/pages/7.8.adoc

@@ -75,6 +75,7 @@
 |compose
 |decompose
 |sys_context
+|lnnvl
 |====
 
 == Built-in function descriptions
@@ -1149,3 +1150,11 @@ ivorysql=# select sys_context('USERENV','DB_NAME');
 (1 row)
 
 ```
+=== `lnnvl` function
+function: return TRUE if the condition is FALSE or UNKNOWN (NULL), and FALSE if the condition is TRUE. It is commonly used in a WHERE clause to also include rows whose condition evaluates to NULL. The test cases are as follows:
+
+Query the rows where the value is not equal to 10 or is NULL:
+
+```
+select * from t where lnnvl(a = 10);
+```