Improve documentation

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

libc/calls/fcntl.c

@@ -30,6 +30,7 @@
  * @param arg can be FD_CLOEXEC, etc. depending
  * @return 0 on success, or -1 w/ errno
  * @asyncsignalsafe
+ * @syscall
  */
 int fcntl(int fd, int cmd, ...) {
   va_list va;