followup: rename to print_snapshot_tree, add comment and rename $res param

followup: rename to print_snapshot_tree, add comment and rename $res param

This was taken from a CLI helper, there $res is a common parameter
name which denotes that it's the res from the API call the CLI
command bases on. But here that makes no sense and is not really
telling about what the value(s) of $res could be. Further explain
that with a comment.

As this also prints uncoditionally to STDOUT let's say so through the
method name.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>