docs: add say_stream notes to the sending messages page

docs/english/_sidebar.json

@@ -85,11 +85,7 @@
 			"tools/bolt-python/concepts/token-rotation"
 		]
 	},
-	{
-		"type": "category",
-		"label": "Experiments",
-		"items": ["tools/bolt-python/experiments"]
-	},
+	"tools/bolt-python/experiments",
 	{
 		"type": "category",
 		"label": "Legacy",

docs/english/experiments.md

@@ -8,6 +8,7 @@ We love feedback from our community, so we encourage you to explore and interact
 
 ## Available experiments
 * [Agent listener argument](#agent)
+* [`say_stream` utility](#say-stream)
 
 ## Agent listener argument {#agent}
 
@@ -31,4 +32,47 @@ def handle_mention(agent: BoltAgent):
 
 ### Limitations
 
-The `chat_stream()` method currently only works when the `thread_ts` field is available in the event context (DMs and threaded replies). Top-level channel messages do not have a `thread_ts` field, and the `ts` field is not yet provided to `BoltAgent`.
+The `chat_stream()` method currently only works when the `thread_ts` field is available in the event context (DMs and threaded replies). Top-level channel messages do not have a `thread_ts` field, and the `ts` field is not yet provided to `BoltAgent`.
+
+## `say_stream` utility {#say-stream}
+
+The `say_stream` utility is a listener argument available on `app.event` and `app.message` listeners. 
+
+The `say_stream` utility streamlines calling the Python Slack SDK's [`WebClient.chat_stream`](https://docs.slack.dev/tools/python-slack-sdk/reference/web/client.html#slack_sdk.web.client.WebClient.chat_stream) helper utility by sourcing parameter values from the relevant event payload.
+
+| Parameter | Value |
+|---|---| 
+| `channel_id` | Sourced from the event payload.
+| `thread_ts` | Sourced from the event payload. Falls back to the `ts` value if available.
+| `recipient_team_id` | Sourced from the event `team_id` (`enterprise_id` if the app is installed on an org).
+| `recipient_user_id` | Sourced from the `user_id` of the event.
+
+If neither a `channel_id` or `thread_ts` can be sourced, then the utility will merely be `None`.
+
+### Example {#example}
+
+```py
+import os
+
+from slack_bolt import App, SayStream
+from slack_bolt.adapter.socket_mode import SocketModeHandler
+from slack_sdk import WebClient
+
+app = App(token=os.environ.get("SLACK_BOT_TOKEN"))
+
+@app.event("app_mention")
+def handle_app_mention(client: WebClient, say_stream: SayStream):
+  stream = say_stream()
+  stream.append(markdown_text="Someone rang the bat signal!")
+  stream.stop()
+
+@app.message("")
+def handle_message(client: WebClient, say_stream: SayStream):
+  stream = say_stream()
+
+  stream.append(markdown_text="Let me consult my *vast knowledge database*...)
+  stream.stop()
+
+if __name__ == "__main__":
+  SocketModeHandler(app, os.environ.get("SLACK_APP_TOKEN")).start()
+```