Improve indentation/wrapping for named vectors/lists in `\usage{}` blocks

[simpar1471]

The issue

When writing a function with a long argument (i.e. one that spans several lines), the .Rd file produced performs no formatting on the the argument, leading to less-comprehensible documentation).

Consider this dummy function, where in the raw R code there are named vectors/lists which are indented for legibility:

#' An example function for this issue
#' @param long_arg A named list
myfunction <- function(
  long_arg_vctr = c(one = "1",
                    two = "2",
                    three = "3",
                    four = "4",
                    five = "5",
                    six = "6",
                    seven = "7"),
  long_arg_list = list(one = c("foo", "bar"),
                       two = "baz",
                       three = c("a", "b", "c", "d"),
                       four = c(1, 2, 3, 4))
) {
  "Placeholder text so this function runs"
}

The resulting .Rd file loses that indentation, which reduces legibility:

% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/growth_classify.R
\name{myfunction}
\alias{myfunction}
\title{An example function for this issue}
\usage{
myfunction(
  long_arg_vctr = c(one = "1", two = "2", three = "3", four = "4", five = "5", six = "6",
    seven = "7"),
  long_arg_list = list(one = c("foo", "bar"), two = "baz", three = c("a", "b", "c", "d"),
    four = c("1", "2", "3", "4"))
)
}
\arguments{
\item{long_arg_vctr}{A named vector}

\item{long_arg_list}{A named list}
}
\description{
An example function for this issue
}

A nicer(?) outcome

I think it may be prefereable to format the \usage{} blocks to format named vectors/lists more nicely, so that they appear more legible in the final .Rd output. Something like this, perhaps:

% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/growth_classify.R
\name{myfunction}
\alias{myfunction}
\title{An example function for this issue}
\usage{
myfunction(
  long_arg_vctr = c(one = "1",
                    two = "2",
                    three = "3",
                    four = "4",
                    five = "5",
                    six = "6",
                    seven = "7"),
  long_arg_list = list(one = c("foo", "bar"),
                       two = "baz",
                       three = c("a", "b", "c", "d"),
                       four = c(1, 2, 3, 4))
)
}
\arguments{
\item{long_arg_vctr}{A named vector}

\item{long_arg_list}{A named list}
}
\description{
An example function for this issue
}