vswitch: Add missing documentation for "ct_flush" capability

Fixes: 08146bf ("openflow: Add extension to flush CT by generic match.")
Signed-off-by: Ales Musil <amusil@redhat.com>

vswitchd/vswitch.xml

@@ -6301,6 +6301,12 @@ ovs-vsctl add-port br0 p0 -- set Interface p0 type=patch options:peer=p1 \
         translated to an ephemeral port. If there is no collision, no SNAT
         is performed.
       </column>
+      <column name="capabilities" key="ct_flush"
+              type='{"type": "boolean"}'>
+        True if the datapath supports CT flush OpenFlow Nicira extension
+        called <code>NXT_CT_FLUSH</code>. The <code>NXT_CT_FLUSH</code>
+        extensions allows to flush CT entries based on specified parameters.
+      </column>
     </group>
 
     <group title="Common Columns">