libnetlink: add caveat

There are much better API's to netlink now. Encourage users to look
elsewhere.

man/man3/libnetlink.3

@@ -183,6 +183,8 @@ Initialize the rtnetlink attribute
 with a variable length data value.
 
 .SH BUGS
+This library is meant for internal use, use libmnl for new programs.
+
 The functions sometimes use fprintf and exit when a fatal error occurs.
 This library should be named librtnetlink.