I think it could be summarised as thus, in general terms:

You should never need to write a comment to explain what you're doing. If you feel you have to, rewrite the code until it doesn't need explaining in a comment.

    # this method manipulates dimensions
    def method_x(a, b, c, d)
      # a is the width
      # b is the height
      # c is the depth
      # d is a list of options
      ...
    end

That could obviously be re-written as:

    def manipulate_dimensions(width, height, depth, options)
      ...
    end

However, it may very well be the case you have to explain why some code exists, or why it wasn't done another way:

    def post_to_awkward_api(data)
      # stupid.io doesn't accept HTTP Post params
      # so we have to use a comma separated string
      silly_string = data.join(',')
      ....
    end

    def bug_fix_workaround
      # see http://stackoverflow.com/relevant-question/...
    end

Not that my examples are amazing. But there'll always be the case where another developer (or even yourself) is not privy to the thought process that conceived a particular block of code.