Add a couple of xrefs, since they exist and the Synopsis mentions them.

share/man/man4/ucom.4

@@ -1,4 +1,4 @@
-.\" $NetBSD: ucom.4,v 1.26 2016/09/29 11:38:43 wiz Exp $
+.\" $NetBSD: ucom.4,v 1.27 2019/05/05 00:12:34 pgoyette Exp $
 .\"
 .\" Copyright (c) 1999 The NetBSD Foundation, Inc.
 .\" All rights reserved.
@@ -27,7 +27,7 @@
 .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 .\" POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd September 29, 2016
+.Dd May 5, 2019
 .Dt UCOM 4
 .Os
 .Sh NAME
@@ -88,6 +88,7 @@ are used for dial-out.
 .Sh SEE ALSO
 .Xr tty 4 ,
 .Xr u3g 4 ,
+.Xr uark 4 ,
 .Xr ubsa 4 ,
 .Xr uchcom 4 ,
 .Xr uftdi 4 ,
@@ -95,6 +96,7 @@ are used for dial-out.
 .Xr uhmodem 4 ,
 .Xr uipaq 4 ,
 .Xr ukyopon 4 ,
+.Xr umcs 4 ,
 .Xr umct 4 ,
 .Xr umodem 4 ,
 .Xr uplcom 4 ,