Document new app start transaction behaviour for Flutter

[denrase]

SDK

Flutter SDK

Description

We are currently working on fixing some issues with native app start transactions in getsentry/sentry-dart#2266.

After we are done, we should document that these transactions, whether it is automatic or manual by setting the app start end, only run in the main isolate.

Suggested Solution

I will create PR after the functionality is implemented.

\cc @buenaflor