Document SHOW DATABASES fields that can be NULL (#1562)

Some of the fields in SHOW DATABASES will always be null for databases
that are not running. Update the documentation to reflect this.

Author: mnd999

modules/ROOT/pages/database-administration/standard-databases/listing-databases.adoc

@@ -35,6 +35,9 @@ A database may be described as read-only when using `ALTER DATABASE ... SET ACCE
 
 | databaseID
 | The database unique ID.
+
+A database must be `online` or `deallocating` for this value to be available.
+For other database states the value will be `NULL`.
 | STRING
 
 | serverID
@@ -137,16 +140,24 @@ The value is a string formatted as:
 ----
 {storage engine}-{store format}-{major version}.{minor version}
 ----
+A database must be `online` or `deallocating` for this value to be available.
+For other database states the value will be `NULL`.
 | STRING
 
 | lastCommittedTxn
 | The ID of the last transaction received.
+
+A database must be `online` or `deallocating` for this value to be available.
+For other database states the value will be `NULL`.
 | INTEGER
 
 | replicationLag
 |
 Number of transactions the current database is behind compared to the database on the primary instance.
 The lag is expressed in negative integers. In standalone environments, the value is always `0`.
+
+A database must be `online` or `deallocating` for this value to be available.
+For other database states the value will be `NULL`.
 | INTEGER
 
 |constituents