You can use also the version 12 documentation - just at the top of the page use the version switcher. Anyway, in this case the docs are basically the same and I think I see where the confusion is coming from.
There are two types of relationships in Kentico: named and ad-hoc relationship.
Named relationship - the pages are being added on the page's Properties -> Related pages tab where you can select the relationship name (defined in the Relationship names app).
Ad-hoc relationship - this is the Pages field added to the page type, available on the Form/Content tab. This relationship does not have any name, just GUID.
And now, the confusion comes from the
GetRelatedDocuments method parameter, 'fieldname' - but the method's signature says: "Retrieves a query that selects documents related to the current document with the relationship name given by specified field.". So, the field name is meant a field, which holds the name of the named relationship. But from your description I assume you are using ad-hoc relationship. In this case, you need to use the code mentioned in the other thread or also try using the .InRelationWith parametrization for the document query.