chore(docs): improve documentation with note about initial version an…

…d not needing flag (#6848)

## About the changes

Adds a note about new installations starting with version where most
events and features should have a created_by userid set and thus not
need this configuration option/corresponding scheduled data migration
task

website/docs/using-unleash/deploy/configuring-unleash.md

@@ -121,7 +121,7 @@ unleash.start(unleashOptions);
   - _unleashUrl_ (string) - Used to specify the official URL this instance of Unleash can be accessed at for an end user. Can also be configured through the environment variable `UNLEASH_URL`.
   - _gracefulShutdownEnable_: (boolean) - Used to control if Unleash should shutdown gracefully (close connections, stop tasks,). Defaults to true. `GRACEFUL_SHUTDOWN_ENABLE`
   - _gracefulShutdownTimeout_: (number) - Used to control the timeout, in milliseconds, for shutdown Unleash gracefully. Will kill all connections regardless if this timeout is exceeded. Defaults to 1000ms `GRACEFUL_SHUTDOWN_TIMEOUT`
-  - _enableScheduledCreatedByMigration_: (boolean) - Schedules migrations that fills the field created_by_user_id for past events and features. It does it gradually in chunks and runs every 15 minutes so it doesn't affect the application's performance. Defaults to false. `ENABLE_SCHEDULED_CREATED_BY_MIGRATION`
+  - _enableScheduledCreatedByMigration_: (boolean) - Schedules migrations that fills the field created_by_user_id for past events and features. It does it gradually in chunks and runs every 15 minutes so it doesn't affect the application's performance. Defaults to false. **NOTE:** If your installation started with Unleash v5.8 or later, you should not need this flag! `ENABLE_SCHEDULED_CREATED_BY_MIGRATION`
 - **session** - The session config object takes the following options:
   - _ttlHours_ (number) - The number of hours a user session is allowed to live before a new sign-in is required. Defaults to 48 hours. `SESSION_TTL_HOURS`
   - _clearSiteDataOnLogout_ (boolean) - When `true`, a logout action will return a Clear Site Data response header instructing the browser to clear all cookies on the same domain Unleash is running on. If disabled unleash will only destroy and clear the session cookie. Defaults to _true_. `SESSION_CLEAR_SITE_DATA_ON_LOGOUT`