Minor documentation update in def.c
diff --git a/src/core/def.c b/src/core/def.c
index 70d3e58..8125313 100644
--- a/src/core/def.c
+++ b/src/core/def.c
@@ -20,6 +20,10 @@
* @ingroup sys_layer
* lwIP provides default implementations for non-standard functions.
* These can be mapped to OS functions to reduce code footprint if desired.
+ * All defines related to this section must not be placed in lwipopts.h,
+ * but in arch/cc.h!
+ * These options cannot be \#defined in lwipopts.h since they are not options
+ * of lwIP itself, but options of the lwIP port to your system.
*/
/*
