docs(postgres-notify): add docs for Postgres notification (#955)

Author: georgeh0

docs/docs/ops/sources.md

@@ -313,6 +313,27 @@ The spec takes the following fields:
 *   `included_columns` (`list[str]`, optional): non-primary-key columns to include. If not specified, all non-PK columns are included.
 *   `ordinal_column` (`str`, optional): to specify a non-primary-key column used for change tracking and ordering, e.g. can be a modified timestamp or a monotonic version number. Supported types are integer-like (`bigint`/`integer`) and timestamps (`timestamp`, `timestamptz`).
     `ordinal_column` must not be a primary key column.
+*   `notification` (`cocoindex.sources.PostgresNotification`, optional): when present, enable change capture based on Postgres LISTEN/NOTIFY. It has the following fields:
+    *   `channel_name` (`str`, optional): the Postgres notification channel to listen on. CocoIndex will automatically create the channel with the given name. If omitted, CocoIndex uses `{flow_name}__{source_name}__cocoindex`.
+
+    :::info
+
+    If `notification` is provided, CocoIndex listens for row changes using Postgres LISTEN/NOTIFY and creates the required database objects on demand when the flow starts listening:
+
+    - Function to create notification message: `{channel_name}_n`.
+    - Trigger to react to table changes: `{channel_name}_t` on the specified `table_name`.
+
+    Creation is automatic when listening begins.
+
+    Currently CocoIndex doesn't automatically clean up these objects when the flow is dropped (unlike targets)
+    It's usually OK to leave them as they are, but if you want to clean them up, you can run the following SQL statements to manually drop them:
+
+    ```sql
+    DROP TRIGGER IF EXISTS {channel_name}_t ON "{table_name}";
+    DROP FUNCTION IF EXISTS {channel_name}_n();
+    ```
+
+    :::
 
 ### Schema
 

src/ops/sources/postgres.rs

@@ -789,9 +789,9 @@ impl SourceFactoryBase for Factory {
         .await?;
 
         let notification_ctx = spec.notification.map(|spec| {
-            let channel_name = spec
-                .channel_name
-                .unwrap_or_else(|| format!("coco_{}__{}", context.flow_instance_name, source_name));
+            let channel_name = spec.channel_name.unwrap_or_else(|| {
+                format!("{}__{}__cocoindex", context.flow_instance_name, source_name)
+            });
             NotificationContext {
                 function_name: format!("{channel_name}_n"),
                 trigger_name: format!("{channel_name}_t"),