Documentation

Address suggested changes to documentation, as kindly provided in https://github.com/ropensci/onboarding/issues/87

[X] correct use of roxygen links, code, emphasis, and seeAlso (\code{\link[visdat]{vis_miss}}) (https://github.com/njtierney/visdat/commit/05ab39273408bad8c6d3c5ff447f1da7849694ec)
[X] add return return value in roxygen, e.g., @return A \code{ggplot} object displaying the type of values in the data frame and the position of any missing values. (https://github.com/njtierney/visdat/commit/05ab39273408bad8c6d3c5ff447f1da7849694ec)
[X] Update vis_compare documentation, change from list of ideas to description (https://github.com/njtierney/visdat/commit/dbdbb7e39fe9013108382aa0bff99dbe13dac4ce)
[X] vis_guess, fix first sentence (https://github.com/njtierney/visdat/commit/b1e33d7b213f677d606ab14fcb641eec49dc0b0c)
[X] Add content to roxygen file for visdat so that ?visdat provides documentation (https://github.com/njtierney/visdat/commit/fb29a39c90934a3507b43e9450328bf459e0b3f7)

ropensci / visdat