private void addDocumentFieldSet() {
        for (Field docField : schema.getDocument().fieldSet()) {
            if (docField instanceof SDField && ((SDField) docField).isExtraField()) {
                continue; // skip
            }
            schema.fieldSets().addBuiltInFieldSetItem(DocumentType.DOCUMENT, docField.getName());
        }
    }
Adds field sets for 1) fields defined inside document type 2) fields inside search but outside document

@author Vegard Havdal
config-model/src/main/java/com/yahoo/schema/processing/BuiltInFieldSets.java