Make it clear that env vars must be simple

Closes #20169

Signed-off-by: Doug Davis <dug@us.ibm.com>

docs/reference/commandline/run.md

@@ -220,7 +220,8 @@ system's interfaces.
 
     $ docker run -e MYVAR1 --env MYVAR2=foo --env-file ./env.list ubuntu bash
 
-This sets environmental variables in the container. For illustration all three
+This sets simple (non-array) environmental variables in the container. For
+illustration all three
 flags are shown here. Where `-e`, `--env` take an environment variable and
 value, or if no `=` is provided, then that variable's current value is passed
 through (i.e. `$MYVAR1` from the host is set to `$MYVAR1` in the container).