docs: mention C89 syntax preferences

* docs/hacking.html.in (Code formatting): Document that // comment
and declaration-after-statement are discouraged.
* HACKING: Regenerate.

HACKING

@@ -127,6 +127,12 @@ Note that sometimes you'll have to post-process that output further, by piping
 it through "expand -i", since some leading TABs can get through. Usually
 they're in macro definitions or strings, and should be converted anyhow.
 
+Libvirt requires a C99 compiler for various reasons. However, most of the code
+base prefers to stick to C89 syntax unless there is a compelling reason
+otherwise. For example, it is preferable to use "/* */" comments rather than
+"//". Also, when declaring local variables, the prevailing style has been to
+declare them at the beginning of a scope, rather than immediately before use.
+
 
 Curly braces
 ============

docs/hacking.html.in

@@ -152,6 +152,16 @@
       anyhow.
     </p>
 
+    <p>
+      Libvirt requires a C99 compiler for various reasons.  However,
+      most of the code base prefers to stick to C89 syntax unless
+      there is a compelling reason otherwise.  For example, it is
+      preferable to use <code>/* */</code> comments rather
+      than <code>//</code>.  Also, when declaring local variables, the
+      prevailing style has been to declare them at the beginning of a
+      scope, rather than immediately before use.
+    </p>
+
 
     <h2><a name="curly_braces">Curly braces</a></h2>