add info that Flutter SR is available on iOS and Android only currently (#11888)

Author: buenaflor

docs/platforms/flutter/session-replay/index.mdx

@@ -7,6 +7,8 @@ description: "Learn how to enable the Mobile Session Replay Beta in your app."
 
 <Note>
 
+Flutter Session Replay is available on **iOS** and **Android**.
+
 Mobile support for Session Replay is in Beta. Features available in Beta are still work-in-progress and may have bugs. We recognize the irony.
 
 If you have any questions, feedback or would like to report a bug, please open a [GitHub issue](https://github.com/getsentry/sentry-dart/issues/new?template=BUG_REPORT.yml) with a link to a relevant replay in Sentry if possible.