Well this is not actually a critical issue :) but it seems that over the years a few different styles piled up and maybe stating clearly something about this will help to gain consistency in the future.

What I wrote in the HACKING file is more or less what seems to me the most common habits, but of course it's only a proposal and could be integrate/expanded/rewritten. The last point is the one I'm most unsure (maybe no space between function name and parenthesis is equally common).

A patch for emacs modelines will follow, too, hoping vim users won't complain ;)