GH-3872: Docs for PostgresSubscribableChannel

Fixes https://github.com/spring-projects/spring-integration/issues/3872

* Add documentation for `PostgresSubscribableChannel` and related components
* Add links, example and what's new section.
* Docs clean up

src/reference/asciidoc/jdbc.adoc

@@ -480,6 +480,8 @@ Instead, if possible, consider using either JMS- or AMQP-backed channels instead
 For further reference, see the following resource:
 
 * https://mikehadlow.blogspot.com/2012/04/database-as-queue-anti-pattern.html[The Database As Queue Anti-Pattern].
+
+If you are still planning to use your database as a queue, consider using PostgreSQL and its notification mechanism which is described <<postgresql-push,in a subsequent section>>.
 ====
 
 ===== Concurrent Polling
@@ -575,6 +577,57 @@ The message data for a persistent channel is keyed in the store on the channel n
 Consequently, if the channel names are not globally unique, the channels can pick up data that is not intended for them.
 To avoid this danger, you can use the message store `region` to keep data separate for different physical channels that have the same logical name.
 
+
+[[postgresql-push]]
+==== PostgreSQL: Receiving Push Notifications
+
+PostgreSQL offers a listen and notification framework for receiving push notifications upon database table manipulations.
+Spring Integration leverages this mechanism (starting with version 6.0) to allow for receiving push notifications when new messages are added to a `JdbcChannelMessageStore`.
+When using this feature, a database trigger must be defined, which can be found as part of the comments of the `schema-postgresql.sql` file which is included in the JDBC module of Spring Integration.
+
+Push notifications are received via the `PostgresChannelMessageTableSubscriber` class which allows its subscribers to receive a callback upon the arrival of new messages for any given `region` and `groupId`.
+These notifications are received even if a message was appended on a different JVM, but to the same database.
+The `PostgresSubscribableChannel` implementation uses a `PostgresChannelMessageTableSubscriber.Subscription` contract to pull messages from the store as a reaction for notification from the mentioned `PostgresChannelMessageTableSubscriber` notifications.
+
+For example, push notifications for `some group` can be received as follows:
+
+====
+[source,java]
+----
+@Bean
+public JdbcChannelMessageStore messageStore(DataSource dataSource) {
+    JdbcChannelMessageStore messageStore = new JdbcChannelMessageStore(dataSource);
+    messageStore.setChannelMessageStoreQueryProvider(new PostgresChannelMessageStoreQueryProvider());
+    return messageStore;
+}
+
+@Bean
+public PostgresChannelMessageTableSubscriber subscriber(
+      @Value("${spring.datasource.url}") String url,
+      @Value("${spring.datasource.username}") String username,
+      @Value("${spring.datasource.password}") String password) {
+    return new PostgresChannelMessageTableSubscriber(() ->
+        DriverManager.getConnection(url, username, password).unwrap(PgConnection.class));
+}
+
+@Bean
+public PostgresSubscribableChannel channel(
+    PostgresChannelMessageTableSubscriber subscriber,
+    JdbcChannelMessageStore messageStore) {
+  return new PostgresSubscribableChannel(messageStore, "some group", subscriber);
+}
+----
+====
+
+[IMPORTANT]
+====
+Any active `PostgresChannelMessageTableSubscriber` occupies an exclusive JDBC `Connection` for the duration of its active life cycle.
+It is therefore important that this connection does not originate from a pooling `DataSource`.
+Such connection pools do normally expect that issued connections are closed within a predefined timeout window.
+
+For this need of an exclusive connection, it is also recommended that a JVM only runs a single `PostgresChannelMessageTableSubscriber` which can be used to register any number of subscriptions.
+====
+
 [[stored-procedures]]
 === Stored Procedures
 

src/reference/asciidoc/whats-new.adoc

@@ -17,23 +17,34 @@ In general the project has been moved to Java 17 baseline and migrated from Java
 [[x6.0-new-components]]
 === New Components
 
+[[x6.0-mqtt]]
+==== MQTT ClientManager
+
 A new MQTT `ClientManager` has been added to support a reusable MQTT connection across different channel adapters.
 See <<./mqtt.adoc#mqtt-shared-client,Shared MQTT Client Support>> for more information.
 
 [[x6.0-graphql]]
-=== GraphQL Support
+==== GraphQL Support
 
 The GraphQL support has been added.
 See <<./graphql.adoc#graphql,GraphQL Support>> for more information.
 
 [[x6.0-smb]]
-=== SMB Support
+==== SMB Support
 
 SMB support has been added from the Spring Integration Extensions project.
 The Java DSL (see `org.springframework.integration.smb.dsl.Smb` factory) also has been added to this module.
 An `SmbStreamingMessageSource` and `SmbOutboundGateway` implementation are introduced.
 See <<./smb.adoc#smb,SMB Support>> for more information.
 
+[[x6.0-jdbc]]
+==== PostgreSQL Push Notification
+
+A `PostgresSubscribableChannel` allows to receive push notifications via `PostgresChannelMessageTableSubscriber` upon new messages add to the `JdbcChannelMessageStore`.
+
+See <<./jdbc.adoc#postgresql-push,PostgreSQL: Receiving Push Notifications>> for more information.
+
+
 [[x6.0-general]]
 === General Changes