meta: Backfill v9 migration guide items (#14459)

Backfills items to the draft migration guide for v9 that we missed

- #14433
- #14454

Author: lforst

docs/migration/draft-v9-migration-guide.md

@@ -4,19 +4,20 @@
 
 ## General
 
+- **Returning `null` from `beforeSendSpan` span is deprecated.**
 - **Passing `undefined` to `tracesSampleRate` / `tracesSampler` / `enableTracing` will be handled differently in v9**
 
-In v8, a setup like the following:
+  In v8, a setup like the following:
 
-```ts
-Sentry.init({
-  tracesSampleRate: undefined,
-});
-```
+  ```ts
+  Sentry.init({
+    tracesSampleRate: undefined,
+  });
+  ```
 
-Will result in tracing being _enabled_, although no spans will be generated.
-In v9, we will streamline this behavior so that passing `undefined` will result in tracing being disabled, the same as not passing the option at all.
-If you are relying on `undefined` being passed in and having tracing enabled because of this, you should update your config to set e.g. `tracesSampleRate: 0` instead, which will also enable tracing in v9.
+  Will result in tracing being _enabled_, although no spans will be generated.
+  In v9, we will streamline this behavior so that passing `undefined` will result in tracing being disabled, the same as not passing the option at all.
+  If you are relying on `undefined` being passed in and having tracing enabled because of this, you should update your config to set e.g. `tracesSampleRate: 0` instead, which will also enable tracing in v9.
 
 ## `@sentry/utils`