IfAPI: Update ifnet(9) man page

Add starting documentation of the IfAPI to the ifnet(9) page.  This
replaces the existing struct ifnet documentation, since it's to be
treated as opaque now.

Reviewed by:	#manpages, adrian, ziaee
Differential Revision: https://reviews.freebsd.org/D47931

Author: chmeeedalf

share/man/man9/Makefile

@@ -1373,6 +1373,9 @@ MLINKS+=ifnet.9 if_addmulti.9 \
 	ifnet.9 ifqueue.9 \
 	ifnet.9 ifunit.9 \
 	ifnet.9 ifunit_ref.9
+# IfAPI
+MLINKS+=ifnet.9 ifapi.9
+
 MLINKS+=insmntque.9 insmntque1.9
 MLINKS+=intr_event.9 intr_event_add_handler.9 \
 	intr_event.9 intr_event_create.9 \