fix QuotasOperator javadoc (strimzi#11372)

Signed-off-by: see-quick <maros.orsak159@gmail.com>

Author: see-quick

user-operator/src/main/java/io/strimzi/operator/user/operator/QuotasOperator.java

@@ -200,7 +200,9 @@ private static KafkaUserQuotas emptyQuotas()   {
     }
 
     /**
-     * @return Set with all usernames which have some ACLs set
+     * Returns the set of usernames that have quota configurations stored in the cache.
+     *
+     * @return A CompletionStage containing a Set of usernames with quotas
      */
     @Override
     public CompletionStage<Set<String>> getAllUsers() {