Document defaults for resources' optional properties

[khionu]

The documentation doesn't include the default values for the following properties:

• tailscale_device_key.key_expiry_disabled
• tailscale_tailnet_key.reusable
• tailscale_tailnet_key.ephemeral
• tailscale_tailnet_key.preauthorized
• tailscale_tailnet_key.expiry
• tailscale_tailnet_key.description

[khionu]

Thank you very much!