chore(python): Rename start_span parameter (#14446)

We'll be changing the name of this parameter, updating.

Author: sentrivana

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

@@ -94,7 +94,7 @@ with sentry_sdk.start_span(name="parent"):
                     ...
 ```
 
-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()`.
+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_as_child_span=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`, `custom_sampling_context` (see below). Use `sentry_sdk.continue_trace()` for propagating trace data.