updated documentation: roclet names, seealso

... because I often find myself looking up the arguments to roclets, the goal is for `c("collate", "namespace", "rd")` to show up automatically when I type `'document(' -> Tab` in Rstudio.

While here, I also added references to roxygen2 in function description and @seealso
Merge pull request #485 from dlebauer/patch-4

updated documentation: roclet names, seealso