umb.4: Improve synposis, add netmap

Rewrite synopsis in a more consistent style, and add "device netmap",
In FreeBSD 15, netmap is included in GENERIC/{amd64,arm64,powerpc64},
but it still needs to be used in FreeBSD 14 or custom kernels which
don't include GENERIC.

MFC after:		umb does not mfc
Reviewed by:		gbe
Differential Revision:	https://reviews.freebsd.org/D51732

Author: concussious

share/man/man4/umb.4

@@ -17,34 +17,34 @@
 .\"
 .\" $NetBSD: umb.4,v 1.4 2019/08/30 09:22:17 wiz Exp $
 .\"
-.Dd May 11, 2025
+.Dd August 4, 2025
 .Dt UMB 4
 .Os
 .Sh NAME
 .Nm umb
 .Nd USB Mobile Broadband Interface Model (MBIM) cellular modem driver
 .Sh SYNOPSIS
-To compile this driver into the kernel,
-place the following lines in your
-kernel configuration file:
-.Bd -ragged -offset indent
+.Cd "device netmap"
 .Cd "device usb"
 .Cd "device umb"
-.Ed
 .Pp
-Alternatively, to load the driver as a
-module at boot time, place the following line in
+In
 .Xr loader.conf 5 :
-.Bd -literal -offset indent
-umb_load="YES"
-.Ed
-.Pp
-If neither of the above is done, the driver will automatically be loaded
-by devd(8) when the device is connected.
+.Cd umb_load="YES"
 .Sh DESCRIPTION
 The
 .Nm
 driver provides support for USB MBIM devices.
+If the appropriate hardware is detected,
+the driver will be loaded automatically by
+.Xr devmatch 8 .
+To load the driver manually,
+.Cm load
+it in
+.Xr loader.conf 5
+or at the
+.Xr loader 8
+prompt.
 .Pp
 MBIM devices establish connections via cellular networks such as
 GPRS, UMTS, and LTE.