Add tcp_ecn option to config.ini [freebsd.boot] section

Document the ECN switch introduced by CM4 (VIMAGE). Default 0=disabled
for zero-regression; 1=passive ECN (upstream default).

Author: jfb8856606

config.ini

@@ -319,6 +319,10 @@ hz=100
 # You can increase this value according to your app.
 fd_reserve=1024
 
+# TCP ECN (Explicit Congestion Notification).
+# 0=disable (default, zero-regression); 1=passive ECN (upstream default).
+#tcp_ecn=0
+
 kern.ipc.maxsockets=262144
 
 net.inet.tcp.syncache.hashsize=4096