Improve documentation

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

libc/calls/chown.c

@@ -33,6 +33,7 @@
  * @see /etc/passwd for user ids
  * @see /etc/group for group ids
  * @asyncsignalsafe
+ * @syscall
  */
 int chown(const char *pathname, uint32_t uid, uint32_t gid) {
   if (!pathname) return efault();