Improve clarity of environment variables description

Changed the environment variables section description from:
"The following list of environment variables are supported by the `docker` command line:"

To:
"The following environment variables control the behavior of the `docker` command-line client:"

This makes it clearer that these variables control Docker's behavior, and uses the more precise term "command-line client" instead of "command line".

This change aligns with docker/cli#6602.

Signed-off-by: Andrew Hopp <[email protected]>

Author: ahopp

data/engine-cli/docker.yaml

@@ -29,8 +29,7 @@ long: |-
 
     ### Environment variables
 
-    The following list of environment variables are supported by the `docker` command
-    line:
+    The following environment variables control the behavior of the `docker` command-line client:
 
     | Variable                      | Description                                                                                                                                                                                                                                                       |
     | :---------------------------- |:------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|