provider/digitalocean: Document defaults

Author: jen20

website/source/docs/providers/do/r/droplet.html.markdown

@@ -32,9 +32,11 @@ The following arguments are supported:
 * `name` - (Required) The droplet name
 * `region` - (Required) The region to start in
 * `size` - (Required) The instance size to start
-* `backups` - (Optional) Boolean controlling if backups are made.
-* `ipv6` - (Optional) Boolean controlling if IPv6 is enabled.
-* `private_networking` - (Optional) Boolean controlling if private networks are enabled.
+* `backups` - (Optional) Boolean controlling if backups are made. Defaults to
+   false.
+* `ipv6` - (Optional) Boolean controlling if IPv6 is enabled. Defaults to false.
+* `private_networking` - (Optional) Boolean controlling if private networks are
+   enabled. Defaults to false.
 * `ssh_keys` - (Optional) A list of SSH IDs or fingerprints to enable in
    the format `[12345, 123456]`. To retrieve this info, use a tool such
    as `curl` with the [DigitalOcean API](https://developers.digitalocean.com/#keys),