doc/developer.texi: Add variadic macros to allowed C language features

They are used in several places like CBS.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

doc/developer.texi

@@ -131,6 +131,9 @@ compound literals (@samp{x = (struct s) @{ 17, 23 @};}).
 @item
 for loops with variable definition (@samp{for (int i = 0; i < 8; i++)});
 
+@item
+Variadic macros (@samp{#define ARRAY(nb, ...) (int[nb + 1])@{ nb, __VA_ARGS__ @}});
+
 @item
 Implementation defined behavior for signed integers is assumed to match the
 expected behavior for two's complement. Non representable values in integer