Add clarification that database field depends on the security event (neo4j#1904)

[Trello
card](https://trello.com/c/e1PTFRXS/469-logging-of-databasename-for-securitylog-is-not-for-all-events#)

Author: renetapopova

modules/ROOT/pages/monitoring/logging.adoc

@@ -729,6 +729,7 @@ The following information is available in the JSON format:
 
 | database
 | The database name the command is executed on.
+This field is optional and thus will not be populated for all security events.
 
 | username
 | The user connected to the security event.