Add migration docs

docs/migration/v8-to-v9.md

@@ -88,6 +88,8 @@ In v9, an `undefined` value will be treated the same as if the value is not defi
 
 - The `requestDataIntegration` will no longer automatically set the user from `request.user`. This is an express-specific, undocumented behavior, and also conflicts with our privacy-by-default strategy. Starting in v9, you'll need to manually call `Sentry.setUser()` e.g. in a middleware to set the user on Sentry events.
 
+- The `tracesSampler` hook will no longer be called for _every_ span. Instead, it will only be called for "root spans". Root spans are spans that have no local parent span. Root spans may however have incoming trace data from a different service, for example when using distributed tracing.
+
 ### `@sentry/browser`
 
 - The `captureUserFeedback` method has been removed. Use `captureFeedback` instead and update the `comments` field to `message`.