3.4.15 update

This option tells makeskel to create an update le: it will read description les (section 3.2.2, page 30) and will only create documentation nodes for identiers which do not yet have a documentation node in the read documentation les. The output le in this case can be merged with one (or more) of the documentation les: it's name should not appear in the list of existing documentation les. The makeskel program will do some elementary checks on this.