Document BC break with $secret parameter introduction

Author: wouterj

UPGRADE-6.4.md

@@ -198,7 +198,7 @@ Security
  * [BC break] `UserValueResolver` no longer implements `ArgumentValueResolverInterface`
  * [BC break] Make `PersistentToken` immutable
  * Deprecate accepting only `DateTime` for `TokenProviderInterface::updateToken()`, use `DateTimeInterface` instead
- * Deprecate calling the constructor of `DefaultLoginRateLimiter` with an empty secret
+ * [BC break] Added required `string $secret` parameter to the constructor of `DefaultLoginRateLimiter`
 
 SecurityBundle
 --------------