ETCDCTL_README: clarify the usage of ETCDCTL_* variables

fixes # 10840

etcdctl/README.md

@@ -16,7 +16,7 @@ ETCDCTL_CERT=/tmp/cert.pem
 ETCDCTL_KEY=/tmp/key.pem
 ```
 
-Prefix flag strings with `ETCDCTL_`, convert all letters to upper-case, and replace dash(`-`) with underscore(`_`).
+Prefix flag strings with `ETCDCTL_`, convert all letters to upper-case, and replace dash(`-`) with underscore(`_`). Note that the environment variables with the prefix `ETCDCTL_` can only be used with the etcdctl global flags. Also, the environment variable `ETCDCTL_API` is a special case variable for etcdctl internal use only.
 
 ## Key-value commands