docs: Update conntrack established state description

Patch a867c01 ("conntrack: Fix conntrack new state") fixes
the userspace conntrack behavior.  This patch updates the
corresponding conntrack state description.

Fixes: a867c01 ("conntrack: Fix conntrack new state")
Reported-by: Roni Bar Yanai <roniba@mellanox.com>
Acked-by: Roni Bar Yanai <roniba@mellanox.com>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: William Tu <u9012063@gmail.com>

lib/meta-flow.xml

@@ -2566,8 +2566,8 @@ actions=clone(load:0->NXM_OF_IN_PORT[],output:123)
 
         <dt><code>est</code> (0x02)</dt>
         <dd>
-          Part of an existing connection.  Set to 1 if this is a committed
-          connection.
+          Part of an existing connection.  Set to 1 if packets of a committed
+          connection have been seen by conntrack from both directions.
         </dd>
 
         <dt><code>rel</code> (0x04)</dt>