public FieldUpdate removeFieldUpdate(String fieldName) {
        Field field = documentType.getField(fieldName);
        return field != null ? removeFieldUpdate(field) : null;
    }
Adds all the field- and field path updates of the given document update to this. If the given update refers to a
different document or document type than this, this method throws an exception.

@param update The update whose content to add to this.
@throws IllegalArgumentException If the {@link DocumentId} or {@link DocumentType} of the given DocumentUpdate
                                 does not match the content of this.
document/src/main/java/com/yahoo/document/DocumentUpdate.java