doc(replay): Add to migration docs

Author: mydea

packages/replay/MIGRATION.md

@@ -57,3 +57,14 @@ If you only imported from `@sentry/replay`, this will not affect you.
 
 It is highly unlikely to affect anybody, but the type `IEventBuffer` was renamed to `EventBuffer` for consistency.
 Unless you manually imported this and used it somewhere in your codebase, 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();
+```