improve comments around example volume growing operations

FAQ.md

@@ -28,9 +28,11 @@ SeaweedFS also assumes there is a default disk type, which is either empty or `"
 To pre-allocate volumes, send a request to the Master server. For example:
 
 ```shell
-# To replica
+# To create more volumes in a given data center
 curl "http://localhost:9333/vol/grow?dataCenter=000&count=4" # returns {"count":4}
+# To create more volumes on a given rack
 curl "http://localhost:9333/vol/grow?rack=example-rack&count=4" # returns {"count":4}
+# To create more volumes on a given volume server
 curl "http://localhost:9333/vol/grow?dataNode=example-node&count=4" # returns {"count":4}
 ```