flutter: update v9 migration doc (#14076)

Add more info to the migration that makes it easier to troubleshoot if
an error happens

Author: buenaflor

docs/platforms/dart/guides/flutter/migration.mdx

@@ -70,7 +70,15 @@ options.beforeSend = (event, hint) {
 
 Native crash handling is now enabled by default for Flutter Desktop. 
 Windows ARM64 uses [breakpad](https://chromium.googlesource.com/breakpad/breakpad) and all other desktop platforms use [crashpad](https://chromium.googlesource.com/crashpad/crashpad).
-You can disable this feature by setting the `SENTRY_NATIVE_BACKEND` environment variable to an empty string.
+You can disable this feature by setting the `SENTRY_NATIVE_BACKEND` environment variable to "none". e.g `export SENTRY_NATIVE_BACKEND=none`
+
+<Alert>
+
+On Linux, compiling your Flutter Desktop app with the crashpad backend can fail if your clang toolchain is out of date.
+ - Update your clang to the latest version, then try again.
+ - If you still encounter errors, please file an issue on our [Sentry Dart GitHub repository](https://github.com/getsentry/sentry-dart/issues/).
+
+</Alert>
 
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