ip: update man pages and usage() for 'ip monitor'

Sync with the current code.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
diff --git a/ip/ipmonitor.c b/ip/ipmonitor.c
index 09a339c..a9ff1e8 100644
--- a/ip/ipmonitor.c
+++ b/ip/ipmonitor.c
@@ -29,7 +29,10 @@
 
 static void usage(void)
 {
-	fprintf(stderr, "Usage: ip monitor [ all | LISTofOBJECTS ]\n");
+	fprintf(stderr, "Usage: ip monitor [ all | LISTofOBJECTS ] [ FILE ]\n");
+	fprintf(stderr, "LISTofOBJECTS := link | address | route | mroute | prefix |\n");
+	fprintf(stderr, "                 neigh | netconf\n");
+	fprintf(stderr, "FILE := file FILENAME\n");
 	exit(-1);
 }