cherry pick the @family  help for rd.Rmd

Hi,
This pull request has a commit (27 November) to enhance the documentation (vignette) on @family tag use.
It adds a heading, gives examples, and notes that the tag can be repeated.

I think it will help learning, and is worth cherry picking.

There are other bits and pieces here that over the months I have thought might be helpful (more informative error messages) but ignore those if you wish (and the @family tag request if you wish also, of course :-)