Improve documentation

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

libc/calls/dup3.c

@@ -33,6 +33,8 @@
  * @param newfd if already assigned, is silently closed beforehand;
  *     unless it's equal to oldfd, in which case dup2() is a no-op
  * @flags can have O_CLOEXEC
+ * @see dup(), dup2()
+ * @syscall
  */
 int dup3(int oldfd, int newfd, int flags) {
   if (oldfd == newfd) return einval();