[KYUUBI #3290] update custom event in settings doc

### _Why are the changes needed?_

#3290
### _How was this patch tested?_

<img width="1214" alt="image" src="https://user-images.githubusercontent.com/13965087/211142727-d56b4ccd-dc26-465a-b4c4-6b04354b37d4.png">

Closes #4120 from zzzzming95/kyuubi-3290.

Closes #3290

45023d9 [佘志铭] update KyuubiConf.scala
c926bc3 [佘志铭] update custom event in settings doc

Authored-by: 佘志铭 <shezhiming@corp.netease.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>

docs/deployment/settings.md

@@ -159,7 +159,7 @@ kyuubi.backend.engine.exec.pool.shutdown.timeout|PT10S|Timeout(ms) for the opera
 kyuubi.backend.engine.exec.pool.size|100|Number of threads in the operation execution thread pool of SQL engine applications|int|1.0.0
 kyuubi.backend.engine.exec.pool.wait.queue.size|100|Size of the wait queue for the operation execution thread pool in SQL engine applications|int|1.0.0
 kyuubi.backend.server.event.json.log.path|file:///tmp/kyuubi/events|The location of server events go for the builtin JSON logger|string|1.4.0
-kyuubi.backend.server.event.loggers||A comma separated list of server history loggers, where session/operation etc events go.<ul> <li>JSON: the events will be written to the location of kyuubi.backend.server.event.json.log.path</li> <li>JDBC: to be done</li> <li>CUSTOM: to be done.</li></ul>|seq|1.4.0
+kyuubi.backend.server.event.loggers||A comma separated list of server history loggers, where session/operation etc events go.<ul> <li>JSON: the events will be written to the location of kyuubi.backend.server.event.json.log.path</li> <li>JDBC: to be done</li> <li>CUSTOM: User-defined event handlers.</li></ul> Note that: Kyuubi supports custom event handlers with the Java SPI. To register a custom event handler, user need to implement a class which is a child of org.apache.kyuubi.events.handler.CustomEventHandlerProvider which has zero-arg constructor.|seq|1.4.0
 kyuubi.backend.server.exec.pool.keepalive.time|PT1M|Time(ms) that an idle async thread of the operation execution thread pool will wait for a new task to arrive before terminating in Kyuubi server|duration|1.0.0
 kyuubi.backend.server.exec.pool.shutdown.timeout|PT10S|Timeout(ms) for the operation execution thread pool to terminate in Kyuubi server|duration|1.0.0
 kyuubi.backend.server.exec.pool.size|100|Number of threads in the operation execution thread pool of Kyuubi server|int|1.0.0
@@ -225,7 +225,7 @@ kyuubi.engine.deregister.exception.messages||A comma separated list of exception
 kyuubi.engine.deregister.exception.ttl|PT30M|Time to live(TTL) for exceptions pattern specified in kyuubi.engine.deregister.exception.classes and kyuubi.engine.deregister.exception.messages to deregister engines. Once the total error count hits the kyuubi.engine.deregister.job.max.failures within the TTL, an engine will deregister itself and wait for self-terminated. Otherwise, we suppose that the engine has recovered from temporary failures.|duration|1.2.0
 kyuubi.engine.deregister.job.max.failures|4|Number of failures of job before deregistering the engine.|int|1.2.0
 kyuubi.engine.event.json.log.path|file:///tmp/kyuubi/events|The location of all the engine events go for the builtin JSON logger.<ul><li>Local Path: start with 'file://'</li><li>HDFS Path: start with 'hdfs://'</li></ul>|string|1.3.0
-kyuubi.engine.event.loggers|SPARK|A comma separated list of engine history loggers, where engine/session/operation etc events go.<ul> <li>SPARK: the events will be written to the spark listener bus.</li> <li>JSON: the events will be written to the location of kyuubi.engine.event.json.log.path</li> <li>JDBC: to be done</li> <li>CUSTOM: to be done.</li></ul>|seq|1.3.0
+kyuubi.engine.event.loggers|SPARK|A comma separated list of engine history loggers, where engine/session/operation etc events go.<ul> <li>SPARK: the events will be written to the spark listener bus.</li> <li>JSON: the events will be written to the location of kyuubi.engine.event.json.log.path</li> <li>JDBC: to be done</li> <li>CUSTOM: User-defined event handlers.</li></ul> Note that: Kyuubi supports custom event handlers with the Java SPI. To register a custom event handler, user need to implement a class which is a child of org.apache.kyuubi.events.handler.CustomEventHandlerProvider which has zero-arg constructor.|seq|1.3.0
 kyuubi.engine.flink.extra.classpath|&lt;undefined&gt;|The extra classpath for the flink sql engine, for configuring location of hadoop client jars, etc|string|1.6.0
 kyuubi.engine.flink.java.options|&lt;undefined&gt;|The extra java options for the flink sql engine|string|1.6.0
 kyuubi.engine.flink.memory|1g|The heap memory for the flink sql engine|string|1.6.0

kyuubi-common/src/main/scala/org/apache/kyuubi/config/KyuubiConf.scala

@@ -1815,7 +1815,12 @@ object KyuubiConf {
         s" <li>JSON: the events will be written to the location of" +
         s" ${SERVER_EVENT_JSON_LOG_PATH.key}</li>" +
         s" <li>JDBC: to be done</li>" +
-        s" <li>CUSTOM: to be done.</li></ul>")
+        s" <li>CUSTOM: User-defined event handlers.</li></ul>" +
+        " Note that: Kyuubi supports custom event handlers with the Java SPI." +
+        " To register a custom event handler," +
+        " user need to implement a class" +
+        " which is a child of org.apache.kyuubi.events.handler.CustomEventHandlerProvider" +
+        " which has zero-arg constructor.")
       .version("1.4.0")
       .serverOnly
       .stringConf
@@ -1833,7 +1838,12 @@ object KyuubiConf {
         " <li>JSON: the events will be written to the location of" +
         s" ${ENGINE_EVENT_JSON_LOG_PATH.key}</li>" +
         " <li>JDBC: to be done</li>" +
-        " <li>CUSTOM: to be done.</li></ul>")
+        " <li>CUSTOM: User-defined event handlers.</li></ul>" +
+        " Note that: Kyuubi supports custom event handlers with the Java SPI." +
+        " To register a custom event handler," +
+        " user need to implement a class" +
+        " which is a child of org.apache.kyuubi.events.handler.CustomEventHandlerProvider" +
+        " which has zero-arg constructor.")
       .version("1.3.0")
       .stringConf
       .transform(_.toUpperCase(Locale.ROOT))