ETCDCTL_README: clarify the usage of ETCDCTL_* variables
fixes # 10840
@@ -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
