Feature/csardi sync appdirs

This is a minor reworking of commits originally due to Gabor Csardi to be compatible with my current master. They correspond to

* Docs: `<appname>` -> `<AppName>` to avoid confusion …	9a8687b
* Docs: comment on appauthor -> appname fallback …	a775800
* Only use version if appname is not NULL …	7512326

The reason the commits could not be applied straight is largely because of roxygen2 4.0.1 wrap settings.