v3.1.1
The available configuration options along with their defaults are now described in 41_snapshots-btrfs_config.
In addition to placing the configuration options to /etc/default/grub, you can also now place them in /etc/grub.d/41_snapshots-btrfs_config.
For example, copy the 41_snapshots-btrfs_config to that location, uncomment and edit the options that you need.