doc/developer: update style guidelines to include for loops with declarations

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>

doc/developer.texi

@@ -128,6 +128,9 @@ designated struct initializers (@samp{struct s x = @{ .i = 17 @};});
 @item
 compound literals (@samp{x = (struct s) @{ 17, 23 @};}).
 
+@item
+for loops with variable definition (@samp{for (int i = 0; i < 8; i++)});
+
 @item
 Implementation defined behavior for signed integers is assumed to match the
 expected behavior for two's complement. Non representable values in integer