Document refcount field better

runtime/caml/io.h

@@ -42,7 +42,7 @@ struct channel {
   char * max;                   /* Logical end of the buffer (for input) */
   void * mutex;                 /* Placeholder for mutex (for systhreads) */
   struct channel * next, * prev;/* Double chaining of channels (flush_all) */
-  int refcount;                 /* For flush_all */
+  int refcount;                 /* Number of custom blocks owning the channel */
   int flags;                    /* Bitfield */
   char buff[IO_BUFFER_SIZE];    /* The buffer itself */
   char * name;                  /* Optional name (to report fd leaks) */