Use \code and \link in documentation

[jeremystan]

E.g., from purrr https://github.com/hadley/purrr/blob/master/R/along.R

#' These functions take the idea of \code{\link{seq_along}} and generalise
#' it to creating lists (\code{list_along}) and repeating values
#' (\code{rep_along}).

[jeremystan]

Also use @seealso And review http://r-pkgs.had.co.nz/man.html

[jeremystan]

Also improve the examples.