CodingGuidelines: 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

@@ -430,6 +430,8 @@ 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.