`clipboard()` is not mentioned as possible value for, e.g., read_csv(file =)

[jennybc]

In c2a334f, I switched to inheriting more docs (specifically for file) from vroom::vroom() as opposed to from readr::datasource(). This is a net positive as it better reflects actual reality!

But it is true that readr offers clipboard() for posssible use with file and that's no longer mentioned (vroom does not export such a function). I can live with this omission for now and maybe forever.

This is a note-to-self to revisit this when I assimilate vroom into readr, which changes the landscape.

[chrislongros]

I opened #1618 with a fix that adds an explicit @param file to read_delim() matching the vroom text plus the clipboard() line.