bgpd: add tcp md5sig documentation for rtr

I missed to add the tcp md5sig bits for rtr to bgpd.conf.5
This is just a copy of the same bits in the neighbor section.

-- 
:wq Claudio

Index: bgpd.conf.5
===================================================================
RCS file: /cvs/src/usr.sbin/bgpd/bgpd.conf.5,v
diff -u -p -r1.255 bgpd.conf.5
--- bgpd.conf.5	2 Jun 2026 08:40:02 -0000	1.255
+++ bgpd.conf.5	26 Aug 2026 12:26:35 -0000
@@ -617,6 +617,16 @@ If not specified, the default
 .Ic port
 is
 .Em 323 .
+.Pp
+.It Ic tcp md5sig password Ar secret
+.It Ic tcp md5sig key Ar secret
+Enable TCP MD5 signatures per RFC 2385.
+The shared secret can either be given as a password or hexadecimal key.
+.Bd -literal -offset indent
+tcp md5sig password mekmitasdigoat
+tcp md5sig key deadbeef
+.Ed
+After changing keys, a session needs to be reset to use the new keys.
 .El
 .El
 .Sh NETWORK AND FLOWSPEC ANNOUNCEMENTS