Improve docs about container env_file on remote machine (#621)

Fix #583
Signed-off-by: Sagi Shnaidman <sshnaidm@redhat.com>

plugins/modules/podman_container.py

@@ -266,6 +266,10 @@ options:
       - Read in a line delimited file of environment variables. Doesn't support
         idempotency. If users changes the file with environment variables it's
         on them to recreate the container.
+        The file must be present on the REMOTE machine where actual podman is
+        running, not on the controller machine where Ansible is executing.
+        If you need to copy the file from controller to remote machine, use the
+        copy or slurp module.
     type: path
   env_host:
     description: