document return values correctly

io/io_fd.3

@@ -4,7 +4,7 @@ io_fd \- prepare descriptor for io_wait
 .SH SYNTAX
 .B #include <io.h>
 
-void \fBio_fd\fP(int64 fd);
+int \fBio_fd\fP(int64 fd);
 .SH DESCRIPTION
 If you want to use io_canread() and io_canwrite() on a descriptor, you
 have to use io_wait() on it first, and io_wait() has to know which
@@ -15,5 +15,7 @@ io_pipe and io_socketpair already call io_fd for you.
 Waiting on descriptors only works for sockets, fifos and pipes.  It may
 also work on devices and TTYs, but that is platform dependent -- you
 should not rely on that.  It does not work on files.
+.SH "RETURN VALUE"
+io_fd returns 1 on success, 0 on error.
 .SH "SEE ALSO"
 io_wait(3), io_wantread(3), io_canread(3), io_eagain(3), io_nonblock(3)