go/zodb: pkgdoc: Stress that objects can reference each other in the database

We are too used to have this for granted with ZODB, but this property of
object databases is not generally universally available in other databases.

go/zodb/zodb.go

@@ -47,6 +47,8 @@
 // Object state data is generally opaque, but is traditionally based on Python
 // pickles in ZODB/py world.
 //
+// An object can reference other objects in the database by their oid.
+//
 //
 // Operations
 //