Doc: provide more clarify to the usage of check perf command

This PR describes the fact that different workloads in the `check perf` command
are different, and the results might vary.

Signed-off-by: Eduardo Patrocinio <epatro@gmail.com>

etcdctl/README.md

@@ -1505,6 +1505,10 @@ CHECK provides commands for checking properties of the etcd cluster.
 
 CHECK PERF checks the performance of the etcd cluster for 60 seconds. Running the `check perf` often can create a large keyspace history which can be auto compacted and defragmented using the `--auto-compact` and `--auto-defrag` options as described below.
 
+Notice that different workload models use different configurations in terms of number of clients and expected throughtput.
+Hence, a workload model may work while another one might fail.
+
+
 RPC: CheckPerf
 
 #### Options

etcdctl/ctlv3/command/check.go

@@ -126,7 +126,7 @@ func NewCheckPerfCommand() *cobra.Command {
 	}
 
 	// TODO: support customized configuration
-	cmd.Flags().StringVar(&checkPerfLoad, "load", "s", "The performance check's workload model. Accepted workloads: s(small), m(medium), l(large), xl(xLarge)")
+	cmd.Flags().StringVar(&checkPerfLoad, "load", "s", "The performance check's workload model. Accepted workloads: s(small), m(medium), l(large), xl(xLarge). Different workload models use different configurations in terms of number of clients and expected throughtput.")
 	cmd.Flags().StringVar(&checkPerfPrefix, "prefix", "/etcdctl-check-perf/", "The prefix for writing the performance check's keys.")
 	cmd.Flags().BoolVar(&autoCompact, "auto-compact", false, "Compact storage with last revision after test is finished.")
 	cmd.Flags().BoolVar(&autoDefrag, "auto-defrag", false, "Defragment storage after test is finished.")