Documentation: Annotate --logger flag
This commit annotates the `--logger` flag to let users know that it is
not available in versions 3.3.x or later.
@@ -313,6 +313,9 @@ The security flags help to [build a secure etcd cluster][security].
## Logging flags
### --logger
+
+**Available from v3.4**
+
+ Specify 'zap' for structured logging or 'capnslog'.
+ default: capnslog
+ env variable: ETCD_LOGGER
