User-friendliness: point to @rdname from @describeIn error (and vice versa?)

[MichaelChirico]

I see a lot of code like this:

#' @describeIn foo

Almost always this means the user intended #' @rdname foo, but the corresponding roxygenize() warning is not very helpful:

@describeIn requires name and description

Unfortunately it's not fully trivial to make this change since the error is coming from a more generalized piece of code:

roxygen2/R/tag-parser.R

Line 103 in a34bdf7

warn_roxy_tag(x, "requires {first} and {second}")