Documentation for tags

[romainfrancois]

It would be useful to have some sort of documentation for each tag, so that e.g. we could show it in completion popups:

Maybe a "doc" attribute for a function related to the tag, e.g.

attr(roxy_tag_parse.roxy_tag_rdname, "doc") <- "Something useful about the @rdname tag"

[hadley]

Also need to record whether or not they take arguments (so IDE knows whether to insert space or not), and might be worth somehow associating with a roclet so we could also use to generate an automated list for roxygen2 docs.