system variables: document tidb_enable_noop_functions controls sql_au…

…to_is_null

Signed-off-by: unconsolable <chenzhipeng2012@gmail.com>

system-variables.md

@@ -595,7 +595,7 @@ Constraint checking is always performed in place for pessimistic transactions (d
     * `CREATE TEMPORARY TABLE` syntax
     * `DROP TEMPORARY TABLE` syntax
     * `START TRANSACTION READ ONLY` and `SET TRANSACTION READ ONLY` syntax
-    * The `tx_read_only`, `transaction_read_only`, `offline_mode`, `super_read_only` and `read_only` system variables
+    * The `tx_read_only`, `transaction_read_only`, `offline_mode`, `super_read_only`, `read_only` and `sql_auto_is_null` system variables
 
 > **Warning:**
 >