Fix DEVNULL docs

[wooster0]

Currently there are no docs for the DEVNULL constant.
That's because there's a line generated at the macro between the comment and the constant:

  # The name of the null device on the host platform. `/dev/null` on UNIX and `NUL`
  # on win32.
  #
  # When this device is opened using `File.open`, read operations will always
  # return EOF, and any data written will be immediately discarded.
  #
  # ```
  # File.open(File::DEVNULL) do |file|
  #   file.puts "this is discarded"
  # end
  # ```

    DEVNULL = "/dev/null"

This fixes it.

[asterite]

The description doesn't match the diff.

[wooster0]

@asterite I'm sorry what do you mean?

[asterite]

Oooh... sorry, the line is in the generated macro, not in the code. I see. This makes sense.