re-clarify two examples

(updates https://github.com/hadley/adv-r/pull/412)

I had to add a special case because `simplify2array` doesn't like zero-length input. (`unlist` returns `NULL`, which is trash as well.)

Also realized that na.rm was defaulted to TRUE, so changed that.
Merge pull request #413 from ajschumacher/patch-90

re-clarify two examples