DOCS/contribute.md: expand on recommended C99 usage

DOCS/contribute.md

@@ -152,7 +152,7 @@ General coding
 --------------
 
 - Use C99. Also freely make use of C99 features if it's appropriate, such as
-  stdbool.h.
+  stdbool.h. (Except VLA and complex number types.)
 - Don't use GNU-only features. In some cases they may be warranted, if they
   are optional (such as attributes enabling printf-like format string checks).
   But in general, standard C99 should be used.