docs: clarify migration and retrieval of durable state based persiste…

…nce ids (#544)

* Update docs/src/main/paradox/migration.md

docs/src/main/paradox/migration.md

@@ -9,6 +9,8 @@ idempotent result. Full rolling update when switching database or Persistence pl
 you can migrate most of the data while the system is online and then have a short full shutdown while
 migrating the remaining data that was written after the previous online migration.
 
+The migration tool is intended to run as a separate, standalone application and should not be part of the same jvm process as the main application is running under.
+
 ## Dependencies
 
 The Akka dependencies are available from Akka's library repository. To access them there, you need to configure the URL for this repository.