Migration: explain that user may make edits in their IDE

Change-Id: I8289aa274a7882a41a5411739999f1db77fc196f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/172000
Auto-Submit: Paul Berry <[email protected]>
Commit-Queue: Paul Berry <[email protected]>
Commit-Queue: Michael Thomsen <[email protected]>
Reviewed-by: Michael Thomsen <[email protected]>

Author: stereotype441

pkg/nnbd_migration/lib/migration_cli.dart

@@ -750,6 +750,9 @@ View the migration suggestions by visiting:
 Use this interactive web view to review, improve, or apply the results.
 When finished with the preview, hit ctrl-c to terminate this process.
 
+If you make edits outside of the web view (in your IDE), use the 'Rerun from
+sources' action.
+
 ''');
 
       // Block until sigint (ctrl-c).