chore: add function synchronous execution doc

Author: altafDevRev

fern/docs/pages/references/functions.mdx

@@ -27,6 +27,58 @@ functions:
 - `name`: It is the function name that should match the corresponding function name in the folder.
 - `description`: It describes the function.
 
+### Synchronous Execution with Synchronization Key
+
+To ensure that function executions are processed in order (FIFO) for a given context (such as a user or conversation), you can enable synchronization support in your function’s manifest and use a synchronization key (sync-id).
+
+#### How it Works
+
+1. **Enable Synchronization in Manifest**
+   - In your function’s manifest, add the field:
+     ```yaml
+     functions:
+      - name: foobar
+        description: Let me execute
+        supports_synchronization: true
+     ```
+   - This tells the platform to use a FIFO queue for this function.
+
+1. **Set the Synchronization Key**
+   - When triggering an automation, include the sync-id in your rego policy along with the event_key. For example:
+     ```rego focus
+     package rego
+
+     output = {"event": event, "event_key": event_key, "sync_id": sync_id} {
+         input.request.method == "POST"
+         event := input.request.body
+         event_key := "user-enter"
+         sync_id := "1234567890"
+     } else = {"response": response }{
+         response := {
+             "status_code": 400
+         }
+     }
+     ```
+
+   - The sync-id groups related executions for ordered processing.
+
+
+3. **Execution Behavior**
+   - If supports_synchronization is true and a sync-id is provided, executions with the same sync-id are processed in order.
+   - If no sync-id is provided, executions are grouped by a random string.
+   - If supports_synchronization is false, the sync-id is ignored and executions are processed as usual.
+
+#### Platform Behavior
+
+- The platform uses the sync-id as the message group for FIFO processing.
+- This ensures that all executions with the same sync-id are processed in order, preventing race conditions or duplicate conversations.
+
+#### Use Cases
+
+- Ensuring order of message processing for chatbots or messaging integrations.
+- Preventing duplicate or out-of-order actions when multiple events are triggered rapidly.
+
+
 
 <Callout intent="note">
 Each function should be registered in `src/function-factory.ts` to be available for execution.