CodingGuidelines: our documents are in AsciiDoc

Before talking about notations such as optional [--option] enclosed
in brackets, state that the documents are in AsciiDoc and processed
into other formats.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -237,6 +237,9 @@ For Python scripts:
 
 Writing Documentation:
 
+ Most (if not all) of the documentation pages are written in AsciiDoc
+ and processed into HTML output and manpages.
+
  Every user-visible change should be reflected in the documentation.
  The same general rule as for code applies -- imitate the existing
  conventions.  A few commented examples follow to provide reference