@Override
    public String toString() {
        return "'" + field.getName() + "' " + valueUpdates;
    }
Adds all the {@link ValueUpdate}s of the given FieldUpdate to this. If the given FieldUpdate refers to a
different {@link Field} than this, this method throws an exception.

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