ifnet(9): Add description of IFCAP_NETMAP

Describe IFCAP_NETMAP adding a cross reference to netmap(4).

Reviewed by:	bcr, 0mp
Approved by:	gnn (mentor)
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D17988

share/man/man9/ifnet.9

@@ -28,7 +28,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 6, 2017
+.Dd November 14, 2018
 .Dt IFNET 9
 .Os
 .Sh NAME
@@ -759,6 +759,9 @@ interfaces (implies
 .Dv IFCAP_TSO ) .
 .It Dv IFCAP_LINKSTATE
 This Ethernet interface supports dynamic link state changes.
+.It Dv IFCAP_NETMAP
+This Ethernet interface supports
+.Xr netmap  4 .
 .El
 .Pp
 The ability of advanced network interfaces to offload certain