patchCodingGuidelines: document // comments

> We do not use // comments in our C code, which is implied by the
> description of multi-line comment rule and its examples, but is not
> explicitly spelled out.  Spell it out.
> 
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
>  Documentation/CodingGuidelines | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
> index b358d6bfb8..507d52b0d3 100644
> --- a/Documentation/CodingGuidelines
> +++ b/Documentation/CodingGuidelines
> @@ -439,6 +439,8 @@ For C programs:
>  	 */
>  	_("Here is a translatable string explained by the above.");
>  
> +   We do not use // comments.
> +
>   - Double negation is often harder to understand than no negation
>     at all.