Add hover to function symbol formals

[renkun-ken]

This PR adds hover to function symbol formals so that it is easier to see the documentation of the function argument on hover, especially when the function has many arguments.

#' @param var1 test
#' @param var2 test2
#' @param var3 sum of test and test2
test <- function(var1 = 0, var2 = 1,
                 var3 = var1 + var2
) {
  var1
  var2
}

#' @param index number
res <- lapply(1:10, function(index) {
  index + 1
})

[randy3k]

LGTM. Thank you for your continuous contribution. I am sorry that I didn’t much time to work on this project recently.