doc(replay): Add to migration docs

Author: mydea

packages/replay/MIGRATION.md

@@ -52,3 +52,14 @@ import something from '@sentry/replay/submodule';
 ```
 
 If you only imported from `@sentry/replay`, this will not affect you.
+
+## Reduce public API of Replay integration (https://github.com/getsentry/sentry-javascript/pull/6407)
+
+The result of `new Replay()` now has a much more limited public API. Only the following methods are exposed:
+
+```js
+const replay = new Replay();
+
+replay.start();
+replay.stop();
+```