Update Python migration guide (#13939)

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## DESCRIBE YOUR PR
Some small updated to the Python SDK 2.x to 3.x migration guide. 

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---------

Co-authored-by: Ivana Kellyer <[email protected]>

Author: antonpirker

docs/platforms/python/migration/2.x-to-3.x.mdx

@@ -83,7 +83,7 @@ Tracing in the Sentry Python SDK `3.x` is powered by [OpenTelemetry](https://ope
 
 Any spans without a parent span will become transactions by default. If you want to avoid promoting a span without a parent to a transaction, you can pass the `only_if_parent=True` keyword argument to `sentry_sdk.start_span()`.
 
-`sentry_sdk.start_transaction()` and `sentry_sdk.start_span()` no longer take the following arguments: `trace_id`, `baggage`, `span_id`, `parent_span_id`. Use `sentry_sdk.continue_trace()` for propagating trace data.
+`sentry_sdk.start_transaction()` and `sentry_sdk.start_span()` no longer take the following arguments: `trace_id`, `baggage`, `span_id`, `parent_span_id`, `custom_sampling_context` (see below). Use `sentry_sdk.continue_trace()` for propagating trace data.
 
 `sentry_sdk.continue_trace()` no longer returns a `Transaction` and is now a context manager. To continue a trace from headers or environment variables, start a new span inside `sentry_sdk.continue_trace()`:
 
@@ -300,6 +300,11 @@ sentry_sdk.init(
 )
 ```
 
+### Threading
+
+The parameter `propagate_hub` has been removed from `ThreadingIntegration`. Use the new `propagate_scope` parameter instead. (If you had `ThreadingIntegration(propagate_hub=True)`, you can remove the parameter.)
+
+
 ### clickhouse-driver
 
 The query being executed is now available under the `db.query.text` span attribute (only if `send_default_pii` is `True`).