system variables: document last_insert_id

system-variables.md

@@ -175,6 +175,13 @@ mysql> SELECT * FROM t1;
 - Unit: Seconds
 - This variable represents the idle timeout of the interactive user session, which is measured in seconds. Interactive user session refers to the session established by calling [`mysql_real_connect()`](https://dev.mysql.com/doc/c-api/5.7/en/mysql-real-connect.html) API using the `CLIENT_INTERACTIVE` option (for example, MySQL shell client). This variable is fully compatible with MySQL.
 
+### last_insert_id
+
+- Scope: SESSION
+- Default value: `0`
+- This variable returns the last `AUTO_INCREMENT` or `AUTO_RANDOM` value generated by an insert statement.
+- The value of `last_insert_id` is the same as the value returned by the function `LAST_INSERT_ID()`.
+
 ### last_plan_from_binding <span class="version-mark">New in v4.0</span>
 
 - Scope: SESSION