Document optional dbms connection parameter

Author: soerenreichardt

doc/modules/ROOT/pages/gds-session.adoc

@@ -64,7 +64,8 @@ You can use the `sessions.estimate()` method to estimate the size required.
 Available algorithm categories are listed in our API https://neo4j.com/docs/graph-data-science-client/{docs-version}/api/sessions/algorithm_category[reference].
 
 - **DBMS connection**.
-This is a `DbmsConnectionInfo` object that contains the URI of an Neo4j instance, a username, and a password.
+This optional parameter is a `DbmsConnectionInfo` object that contains the URI of an Neo4j instance, a username, and a password. It is required if either a graph projection from a database or a write-back to a database is needed.
+If not specified, graphs can be xref:graph-object.adoc#construct[constructed via arrow].
 
 - **TTL**.
 This optional parameter specifies the time-to-live of the session.