Documentation/upgrades: highlight client Node.Expiration field change

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

Documentation/upgrades/upgrade_3_4.md

@@ -26,7 +26,7 @@ Highlighted breaking changes in 3.4.
 +etcd --peer-trusted-ca-file ca-peer.crt
 ```
 
-#### Change in ``pkg/transport`
+#### Change in `pkg/transport`
 
 Deprecated `pkg/transport.TLSInfo.CAFile` field.
 
@@ -45,6 +45,17 @@ if err != nil {
 }
 ```
 
+#### Change in `client/Node.Expiration` field
+
+changed [**`client.Node.Expiration` type from `*time.Time` to `time.Time`**](https://github.com/coreos/etcd/pull/9494) with [regenerated v2 `client`](https://github.com/coreos/etcd/pull/9494), to [include 32-bit fixes](https://github.com/coreos/etcd/issues/9447).
+
+```diff
+type Node struct {
+    ...
+-   Expiration *time.Time `json:"expiration,omitempty"`
++   Expiration time.Time `json:"expiration,omitempty"`
+```
+
 ### Server upgrade checklists
 
 #### Upgrade requirements