Improve documentation

The Cosmo API documentation page is pretty good now
https://justine.lol/cosmopolitan/documentation.html

libc/calls/dup2.c

@@ -29,6 +29,7 @@
  *     unless it's equal to oldfd, in which case dup2() is a no-op
  * @return new file descriptor, or -1 w/ errno
  * @asyncsignalsafe
+ * @syscall
  */
 int dup2(int oldfd, int newfd) {
   if (oldfd == newfd) return newfd;