bhyve.8: Clean up network backends section

- Reformat the format lists, use appropriate mdoc macros for
  readability.
- Add a missing Oxford comma.

MFC after:	2 weeks

usr.sbin/bhyve/bhyve.8

@@ -339,14 +339,34 @@ If
 is not specified, the device emulation has no backend and can be
 considered unconnected.
 .Pp
-Network backends:
-.Bl -tag -width 10n
-.It Ar tapN Ns Oo , Ns Ar mac=xx:xx:xx:xx:xx:xx Oc Ns Oo , Ns Ar mtu=N Oc
-.It Ar vmnetN Ns Oo , Ns Ar mac=xx:xx:xx:xx:xx:xx Oc Ns Oo , Ns Ar mtu=N Oc
-.It Ar netgraph,path=ADDRESS,peerhook=HOOK Ns Oo , Ns Ar socket=NAME Oc Ns Oo , Ns Ar hook=HOOK Oc Ns Oo , Ns Ar mac=xx:xx:xx:xx:xx:xx Oc Ns Oo , Ns Ar mtu=N Oc
-.Pp
+Network backends formats for
+.Ar conf :
+.Sm off
+.Bl -bullet
+.It
+.Xo
+.Cm tap Ar N
+.Op Cm \&,mac= Ar xx:xx:xx:xx:xx:xx
+.Op Cm \&,mtu= Ar N
+.Xc
+.It
+.Xo
+.Cm vmnet Ar N
+.Op Cm \&,mac= Ar xx:xx:xx:xx:xx:xx
+.Op Cm \&,mtu= Ar N
+.Xc
+.It
+.Xo
+.Cm netgraph,path= Ar ADDRESS Cm \&,peerhook= Ar HOOK
+.Op Cm \&,socket= Ar NAME
+.Op Cm \&,hook= Ar HOOK
+.Op Cm \&,mac= Ar xx:xx:xx:xx:xx:xx
+.Op Cm \&,mtu= Ar N
+.Xc
+.El
+.Sm on
 If
-.Ar mac
+.Cm mac
 is not specified, the MAC address is derived from a fixed OUI and the
 remaining bytes from an MD5 hash of the slot and function numbers and
 the device name.
@@ -355,32 +375,35 @@ The MAC address is an ASCII string in
 .Xr ethers 5
 format.
 .Pp
-With virtio-net devices, the
-.Ar mtu
+With
+.Cm virtio-net
+devices, the
+.Cm mtu
 parameter can be specified to inform the guest about the largest MTU
 that should be allowed, expressed in bytes.
 .Pp
-With netgraph backend, the
-.Ar path
+With
+.Cm netgraph
+backend, the
+.Cm path
 and
-.Ar peerhook
+.Cm peerhook
 parameters must be specified to set the destination node and corresponding hook.
 The optional parameters
-.Ar socket
+.Cm socket
 and
-.Ar hook
+.Cm hook
 may be used to set the
 .Xr ng_socket 4
 node name and source hook.
 The
 .Ar ADDRESS ,
-.Ar HOOK
+.Ar HOOK ,
 and
 .Ar NAME
 must comply with
 .Xr netgraph 4
 addressing rules.
-.El
 .Pp
 Block storage devices:
 .Bl -tag -width 10n