add recommended config comment

Author: sebastian-alfers

core/src/main/resources/reference.conf

@@ -371,6 +371,8 @@ akka.persistence.r2dbc {
   # Ignored for H2
   db-timestamp-monotonic-increasing = off
 
+  # Enable this to generate timestamps from the Akka client side instead of using database timestamps.
+  # NTP or similar clock synchronization should be used, but some clock skew between clients is accepted.
   # Ignored for H2
   use-app-timestamp = off