Doc: update etcd configuration detail with config file

Add a reference, sample file link and precedence detail etc. to the
configuration doc.

Documentation/op-guide/configuration.md

@@ -2,9 +2,10 @@
 
 etcd is configurable through a configuration file, various command-line flags, and environment variables.
 
-A reusable configuration file is a YAML file made of one or more command-line flags described below. In order to use this file, specify the file path as a value to the `--config-file` flag. The [sample configuration file][sample-config-file] can be used as a starting point to create a new configuration file as needed.
+A reusable configuration file is a YAML file made with names of one or more command-line flags described below. In order to use this file, specify the file path as a value to the `--config-file` flag. The [sample configuration file][sample-config-file] can be used as a starting point to create a new configuration file as needed.
 
 Options set on the command line take precedence over those from the environment. If a configuration file is provided, other command line flags will be ignored.
+For example, `etcd --config-file etcd.conf.yml.sample --data-dir /tmp` will ignore the `--data-dir` flag.
 
 The format of environment variable for flag `--my-flag` is `ETCD_MY_FLAG`. It applies to all flags.