clarifying note about default databases (#1370)

Author: JPryce-Aklundh

modules/ROOT/pages/introduction/cypher-neo4j.adoc

@@ -18,7 +18,7 @@ Cypher works almost identically between the two editions, but there are key area
 
 | link:{neo4j-docs-base-uri}/operations-manual/current/database-administration/[Multi-database]
 a|
-Any number of user databases.
+A `system` database and any number of user databases.
 a|
 Only `system` and one user database.
 
@@ -41,6 +41,13 @@ Only xref::constraints/managing-constraints.adoc#create-property-uniqueness-cons
 
 |===
 
+[NOTE]
+The `system` database cannot be deleted.
+The default user database, `neo4j`, can be deleted.
+If you want to delete it, first set another database as the default using the procedure link:{neo4j-docs-base-uri}/operations-manual/current/procedures/#procedure_dbms_setDefaultDatabase[`dbms.setDefaultDatabase]` (this must be done from the `system` database).
+For more information, see the link:{neo4j-docs-base-uri}/operations-manual/current/clustering/databases/#cluster-default-database[Operations Manual -> Change the default database].
+
+
 [[neo4j-terminology]]
 == Key Neo4j terminology