Add missing migration guide entries

Author: sentrivana

MIGRATION_GUIDE.md

@@ -7,6 +7,7 @@ Looking to upgrade from Sentry SDK 2.x to 3.x? Here's a comprehensive list of wh
 ### New Features
 
 - Added `add_attachment()` as a top level API, so you can do now: `sentry_sdk.add_attachment(...)` (up until now it was only available on the `Scope`)
+- Added a new SDK option `exclude_span_origins`. Spans with an `origin` from `exclude_span_origins` won't be created. This can be used for example in dual OTel-Sentry setups to filter out spans from specific Sentry instrumentations. Note that using `exclude_span_origins` might potentially lead to surprising results: if, for example, a root span is excluded based on `origin`, all of its children will become root spans, unless they were started with `only_as_child_span=True`.
 
 ### Changed
 
@@ -26,6 +27,7 @@ Looking to upgrade from Sentry SDK 2.x to 3.x? Here's a comprehensive list of wh
 - `sentry_sdk.start_transaction`/`sentry_sdk.start_span` no longer takes the following arguments: `span`, `parent_sampled`, `trace_id`, `span_id` or `parent_span_id`.
 - `sentry_sdk.continue_trace` no longer returns a `Transaction` and is now a context manager.
 - You can no longer change the sampled status of a span with `span.sampled = False` after starting it. The sampling decision needs to be either be made in the `traces_sampler`, or you need to pass an explicit `sampled` parameter to `start_span`.
+- `sentry_sdk.start_span` now takes an optional `only_as_child_span` argument. These spans will not be started if they would be root spans -- they can only exist as child spans.
 - The `Span()` constructor does not accept a `hub` parameter anymore.
 - `Span.finish()` does not accept a `hub` parameter anymore.
 - `Span.finish()` no longer returns the `event_id` if the event is sent to Sentry.