Add some comments.

src/racket/src/schpriv.h

@@ -2208,14 +2208,16 @@ typedef struct {
 
 typedef struct Scheme_Native_Closure_Data {
   Scheme_Inclhash_Object iso; /* type tag only set when needed, but flags always needed */
-  Scheme_Closed_Prim *code;
+  Scheme_Closed_Prim *code; /* When not yet JITted, this is = to scheme_on_demand_jit_code */
   union {
     void *tail_code;                       /* For non-case-lambda */
     mzshort *arities;                      /* For case-lambda */
   } u;
   void *arity_code;
   mzshort max_let_depth; /* In bytes instead of words */
-  mzshort closure_size;
+  mzshort closure_size; /* if this is negative, then this is a
+                           case-lambda, and the real size is
+                           (-closure-size)-1 */
   union {
     struct Scheme_Closure_Data *orig_code; /* For not-yet-JITted non-case-lambda */
     Scheme_Object *name;