Fix and clarify gnuf_printf

include/ucore/ucore_buffer.h

@@ -59,13 +59,16 @@ void uc_gbuf_unref(GBuf *buf);
 int uc_gbuf_append(GBuf *buf,const void *data,size_t len);
 
 /** Do printf formatting on a GBuf.
- * The buffer will grow as needed.
+ * The buffer will grow as needed,
+ * The buffer ensures that the added string will be nul terminated,
+ * however the nul terminator is not counted in the ->used member.
  *
  * @param buf The GBuf
  * @param fmt printf stype formatting string
  * @param ... format arguments
  * 
- * @return The number of chars written, or negative if an error occurs
+ * @return The number of chars written excluding the terminating nul
+ * character , or negative if an error occurs
  */
 int uc_gbuf_printf(GBuf *buf, const char *fmt, ...);