Document `@return` for functions

[willgearty]

CRAN requires that all functions have a @return value (I think they only actually check this for new submissions, but it indicates a good standard that we should follow). Beyond this, it looks like it is now best practice to always return something, even if it's just the first argument, to facilitate piping (although note that ggplot2 still doesn't do this for most of their functions).