Documentation/upgrades: clarify logger flag changes

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

Documentation/upgrades/upgrade_3_3.md

@@ -22,7 +22,7 @@ Changed `--auto-compaction-retention` flag to [accept string values](https://git
 -auto-compaction-retention: 24
 +auto-compaction-retention: "24"
 +# Or
-+# auto-compaction-retention: "24h"
++auto-compaction-retention: "24h"
 ```
 
 #### Changed `etcdserver.EtcdServer.ServerConfig` to `*etcdserver.EtcdServer.ServerConfig`

Documentation/upgrades/upgrade_3_4.md

@@ -34,7 +34,11 @@ Rename [`etcd --log-output` to `--log-outputs`](https://github.com/coreos/etcd/p
 
 ```diff
 -etcd --log-output stderr
-+etcd --log-outputs stderr,a.log
++etcd --log-outputs stderr
+
++# to write logs to stderr and a.log file at the same time
++# only "--logger zap" supports multiple writers
++etcd --logger zap --log-outputs stderr,a.log
 ```
 
 v3.4 adds `etcd --logger zap` support for structured logging and multiple log outputs. Main motivation is to promote automated etcd monitoring, rather than looking back server logs when it starts breaking. Future development will make etcd log as few as possible, and make etcd easier to monitor with metrics and alerts. **`etcd --logger=capnslog` will be deprecated in v3.5**.

Documentation/upgrades/upgrade_3_5.md

@@ -14,10 +14,15 @@ Highlighted breaking changes in 3.5.
 
 #### Deprecated in `etcd --logger capnslog`
 
-v3.4 adds `--logger=zap` to support multiple log outputs and structured logging. **`etcd --logger=capnslog` has been deprecated in v3.5**, and now `--logger=zap` is the default.
+v3.4 defaults to `--logger=zap` in order to support multiple log outputs and structured logging.
+
+**`etcd --logger=capnslog` has been deprecated in v3.5**, and now `--logger=zap` is the default.
 
 ```diff
 -etcd --logger capnslog
++etcd --logger zap --log-outputs stderr
+
++# to write logs to stderr and a.log file at the same time
 +etcd --logger zap --log-outputs stderr,a.log
 ```
 
@@ -31,7 +36,7 @@ v3.4 renamed [`etcd --log-output` to `--log-outputs`](https://github.com/coreos/
 
 ```diff
 -etcd --log-output stderr
-+etcd --log-outputs stderr,a.log
++etcd --log-outputs stderr
 ```
 
 #### Deprecated in `etcd --log-package-levels`