Document that env vars can't be used for config (#6912)

* Document that env vars can't be used for config

Environment variables are not read for config values when starting the
Consul agent. Document this.

website/source/docs/agent/options.html.md

@@ -16,8 +16,7 @@ descriptions.
 Configuration precedence is evaluated in the following order:
 
 1. Command line arguments
-2. Environment Variables
+2. Configuration files
-3. Configuration files
 
 When loading configuration, Consul loads the configuration from files and
 directories in lexical order. For example, configuration file
@@ -41,6 +40,14 @@ configuration reload.
 
 You can test the following configuration options by following the [Getting Started](https://learn.hashicorp.com/consul/getting-started/install?utm_source=consul.io&utm_medium=docs) guides to install a local agent. 
 
+## Environment Variables
+Environment variables **cannot** be used to configure the Consul client. They
+*can* be used when running other `consul` CLI commands that connect with a
+running agent, e.g. `CONSUL_HTTP_ADDR=192.168.0.1:8500 consul members`.
+
+See [Consul Commands](/docs/commands/index.html#environment-variables) for more
+information.
+
 ## <a name="commandline_options"></a>Command-line Options
 
 The options below are all specified on the command-line.