Topic add matched event description (#3406)

Signed-off-by: Barry Xu <barry.xu@sony.com>

source/Concepts/About-Quality-of-Service-Settings.rst

@@ -301,3 +301,22 @@ Developers may subscribe to the following QoS events that are associated with a
 * Requested incompatible QoS
 
   The subscription has encountered a publisher on the same topic that is offering a QoS profile that does not satisfy the requested QoS profile, resulting in no connection between the subscription and that publisher.
+
+
+Matched events
+--------------
+
+In addition to QoS events, matched events can be generated when any publisher and subscription establishes or drops the connection between them.
+Developers may provide each publisher and subscription with callback functions that are triggered by matched events and handle them in a way they see fit, similar to how messages received on a topic are handled.
+
+* matched
+
+  Developers can subscribe to this event with a publisher or a subscription.
+
+  * publisher: this event happens when it finds a subscription which matches the topic and has compatible QoS or a connected subscription is disconnected
+  * subscription: this event happens when it finds a publisher which matches the topic and has compatible QoS or a connected publisher is disconnected
+
+  There are demo code on how to use event.
+
+  * rclcpp: `demo code <https://github.com/ros2/demos/tree/{REPOS_FILE_BRANCH}/demo_nodes_cpp/src/events/matched_event_detect.cpp>`
+  * rclpy: `demo code <https://github.com/ros2/demos/tree/rolling/{REPOS_FILE_BRANCH}/demo_nodes_py/events/matched_event_detect.py>`