DOC: tfo: bump required kernel to linux-3.7

Support for server side TFO was actually introduced in linux-3.7,
linux-3.6 just has client support.

This patch fixes documentation and a code comment about the
kernel requirement. It also fixes a wrong tfo related code
comment in src/proto_tcp.c.
diff --git a/src/proto_tcp.c b/src/proto_tcp.c
index 60b111c..6bd5761 100644
--- a/src/proto_tcp.c
+++ b/src/proto_tcp.c
@@ -1776,7 +1776,7 @@
 #endif
 
 #ifdef TCP_FASTOPEN
-/* parse the "defer-accept" bind keyword */
+/* parse the "tfo" bind keyword */
 static int bind_parse_tfo(char **args, int cur_arg, struct proxy *px, struct bind_conf *conf, char **err)
 {
 	struct listener *l;