Clarify that uc_read_fd always close() the descriptor

include/ucore/read_file.h

@@ -25,6 +25,8 @@ uc_read_file(const char *file_name, size_t *length, size_t max);
  * @see uc_read_file
  * This is similar but reads from a file descriptor,
  * of any type as long as it supports the read() call.
+ *
+ * uc_read_fd close() the fd in all cases before it returns
  */
 char *
 uc_read_fd(int fd, size_t *length, size_t max);