docs: correct the Python and Go event data model tip (#2160)

Correct the streaming-events documentation and its docs-style example to describe the per-event payload types used by Python and Go.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: examon

.github/instructions/docs-style.instructions.md

@@ -49,7 +49,7 @@ When a callout applies to a specific language, put the qualifier as bold text in
 
 ```markdown
 > [!TIP]
-> **(Python / Go)** These SDKs use a single `Data` class/struct with all fields optional.
+> **(Python / Go)** These SDKs use separate, per-event data types.
 ```
 
 ## Lists

docs/features/streaming-events.md

@@ -210,7 +210,7 @@ session.on(AssistantMessageDeltaEvent.class, event ->
 </details>
 
 > [!TIP]
-> **(Python / Go)** These SDKs use a single `Data` class/struct with all possible fields as optional/nullable. Only the fields listed in the tables below are populated for each event type—the rest will be `None` / `nil`.
+> **(Python / Go)** These SDKs use separate, per-event data types (for example, `AssistantMessageDeltaData`), so only the relevant fields exist on each type.
 >
 > [!TIP]
 > **(.NET)** The .NET SDK uses separate, strongly-typed data classes per event (e.g., `AssistantMessageDeltaData`), so only the relevant fields exist on each type.