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".

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

Author: ahopp

docs/reference/commandline/docker.md

@@ -114,8 +114,7 @@ Options:
 
 ### 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                                                                                                                                                                                                                                                       |
 | :---------------------------- |:------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|