How to properly document S4 "[" and “[<-“ methods using roxygen?

I finally figured it out more or less. At least it works now:

#' An S4 class that stores a string.
#' @slot a contains a string
#' @export
setClass("testClass", 
     representation(a="character"))

#' extract parts of testClass
#'
#' @name [
#' @aliases [,testClass-method
#' @docType methods
#' @rdname extract-methods
#'
setMethod("[", signature(x = "testClass", i = "ANY", j="ANY"),
    function (x, i, j, ..., drop){
       print("void function")
    }
)

As of roxygen2 >3.0.0, you no longer need work arounds and only need:

#' Extract parts of testClass.
#'
setMethod("[", signature(x = "testClass", i = "ANY", j="ANY"),
  function (x, i, j, ..., drop){
    print("void function")
  }
)

For what it's worth, in the case of a replacement function, you'll want something like the following:

#' An S4 class that stores a list.
#' @export
    setClass("testClass", 
      representation(a="list"))

#' extract parts of testClass
#'
#' @name [
#' @aliases [,testClass-method
#' @docType methods
#' @rdname extract-methods
setMethod("[", signature(x = "testClass", i = "ANY", j="ANY"),
  function (x, i, j, ..., drop) {
     x@a[i]
  }
)

#' replace names of testClass
#'
#' @name [
#' @aliases [<-,testClass-method
#' @docType methods
#' @rdname extract-methods
setReplaceMethod("names", signature(x = "testClass", value = "ANY"), definition = function (x, value) {
  names(x@a) <- value
  x
})