title: S4 documentation of "[" with 'missing' arguments
body: <p>This question is very similar to <a href="https://stackoverflow.com/questions/4396768/how-to-properly-document-s4-and-methods-using-roxygen">this question</a> but when I try the answer I receive an addition 'NOTE' following <code>R CMD check</code>.   Although it is just a <code>NOTE</code> I would really like to have a completely clean check.</p>
  
  <pre><code>* checking Rd line widths ... NOTE
  Error: 8: no comma in argument list following \S4method
  Execution halted
  </code></pre>
  
  <p>I can get rid of this if I pass all the other parameters (<code>i,j,drop</code>) and document all of them but I don't use them.  It seems to me that it would be a waste to just add in extra documentation when it isn't relevant in this case.</p>
  
  <pre><code>#' An S4 class that stores a list.
  #' @export
  setClass("testClass", 
                   representation(a="list"))
  
  #' Extract parts of testClass.
  #' @param x testClass
  #'
  setMethod("[", signature(x = "testClass"),
              function (x){
                  print("void function")
              }
  )
  </code></pre>
  
  <p>The Rd file resulting in the error:</p>
  
  <pre><code>% Generated by roxygen2 (4.1.1): do not edit by hand
  % Please edit documentation in R/test.R
  \docType{methods}
  \name{[,testClass-method}
  \alias{[,testClass-method}
  \title{Extract all elements}
  \usage{
  \S4method{[}{testClass}(x)
  }
  \arguments{
  \item{x}{testClass}
  }
  \description{
  Extract all elements
  }
  </code></pre>
  
  <p>The following Rd results in no error if I define and document all the arguments</p>
  
  <pre><code>% Generated by roxygen2 (4.1.1): do not edit by hand
  % Please edit documentation in R/test.R
  \docType{methods}
  \name{[,testClass,missing,missing,missing-method}
  \alias{[,testClass,missing,missing,missing-method}
  \title{Extract all elements}
  \usage{
  \S4method{[}{testClass,missing,missing,missing}(x, i, j, drop)
  }
  \arguments{
  \item{x}{testClass}
  
  \item{i}{missing}
  
  \item{j}{missing}
  
  \item{drop}{missing}
  }
  \description{
  Extract all elements
  }
  </code></pre>