[Fix #718] Don't regard annotations as class/module documentation

I've chosed to base the distinction of what is an annotation comment only on the last line. Another alternative is to say that a comment is not a class documentation comment if there is an annotation *anywhere* in it. But I thought that a comment such as https://github.com/bbatsov/rubocop/blob/master/lib/rubocop/cop/style/one_line_conditional.rb#L6-L7 was reasonable.