add short info about conntrack modules (#731)

docs/configexamples/ha.rst

@@ -1,5 +1,7 @@
 :lastproofread: 2021-06-28
 
+.. _example-high-availability:
+
 #############################
 High Availability Walkthrough
 #############################

docs/configuration/system/conntrack.disable

@@ -1,2 +0,0 @@
-conntrack
-#########

docs/configuration/system/conntrack.rst

@@ -0,0 +1,33 @@
+###################
+Connection tracking
+###################
+
+Modules
+-------
+
+.. code-block:: none
+
+    conntrack {
+        modules {
+            ftp
+            h323
+            nfs
+            pptp
+            sip
+            sqlnet
+            tftp
+        }
+    }
+
+Enables ``conntrack`` modules. All modules are now disabled by default, while they
+used to be enabled in previous versions. Enabling the modules ensures backwards
+compatibility — keeping the previous behavior.
+
+In most cases they can be disabled by removing the block of configuration.
+
+.. code-block:: none
+
+    delete system conntrack modules
+
+For some scenarios it is in fact recommended, like in this example:
+:ref:`example-high-availability`.

docs/configuration/system/index.rst

@@ -7,6 +7,7 @@ System
    :maxdepth: 1
    :includehidden:
 
+   conntrack
    console
    flow-accounting
    host-name