ip/tunnel: Document "external" parameter

Add it to ip-link(8) "type gre" output help message
as well as to ip-link(8) page.

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>

ip/link_gre.c

@@ -43,6 +43,7 @@ static void print_usage(FILE *f)
 		"                            [ [no]encap-csum ]\n"
 		"                            [ [no]encap-csum6 ]\n"
 		"                            [ [no]encap-remcsum ]\n"
+		"                            [ external ]\n"
 		"                            [ fwmark MARK ]\n"
 		"                            [ erspan IDX ]\n"
 		"\n"

man/man8/ip-link.8.in

@@ -698,6 +698,8 @@ the following additional arguments are supported:
 .I " mode " { ip6ip | ipip | mplsip | any } "
 ] [
 .BR erspan " \fIIDX "
+] [
+.BR external
 ]
 
 .in +8
@@ -749,6 +751,11 @@ IPv6-Over-IPv4 is not supported for IPIP.
 indicates a 20 bit index/port number associated with the ERSPAN
 traffic's source port and direction.
 
+.sp
+.BR external
+- make this tunnel externally controlled
+.RB "(e.g. " "ip route encap" ).
+
 .in -8
 
 .TP