Fix cbuf documentation
#include <unistd.h>
// To define a circular buffer type of 20 ints:
-// typedef CBUF(int, 20) my_cbuf_t;
+// struct cbuf_int CBUF(int, 20);
//
// data has length CAP + 1 to distinguish empty vs full.
#define CBUF(TYPE, CAP) { \
