Update cluster to match Atlas doc (#372)

provider_encrypt_ebs_volume - update to match underlying API doc.

Author: themantissa

website/docs/r/cluster.html.markdown

@@ -297,7 +297,7 @@ But in order to explicitly change `provider_instance_size_name` comment the `lif
 
 * `provider_disk_iops` - (Optional) The maximum input/output operations per second (IOPS) the system can perform. The possible values depend on the selected `provider_instance_size_name` and `disk_size_gb`.
 * `provider_disk_type_name` - (Optional - Azure Only) Azure disk type of the server’s root volume. If omitted, Atlas uses the default disk type for the selected providerSettings.instanceSizeName.  Example disk types and associated storage sizes: P4 - 32GB, P6 - 64GB, P10 - 128GB, P15 - 256GB, P20 - 512GB, P30 - 1024GB, P40 - 2048GB, P50 - 4095GB.  More information and the most update to date disk types/storage sizes can be located at https://docs.atlas.mongodb.com/reference/api/clusters-create-one/.
-* `provider_encrypt_ebs_volume` - (Optional- AWS ONLY) If enabled, the Amazon EBS encryption feature encrypts the server’s root volume for both data at rest within the volume and for data moving between the volume and the cluster.  **Atlas encrypts all cluster storage and snapshot volumes, securing all cluster data on disk: a concept known as encryption at rest, by default**.
+* `provider_encrypt_ebs_volume` - (Optional - AWS ONLY) The default value is true.  Flag that indicates whether the Amazon EBS encryption feature encrypts the host's root volume for both data at rest within the volume and for data moving between the volume and the cluster. Note: This setting is always enabled for clusters with local NVMe SSDs. **Atlas encrypts all cluster storage and snapshot volumes, securing all cluster data on disk: a concept known as encryption at rest, by default.**.
 * `provider_region_name` - (Optional) Physical location of your MongoDB cluster. The region you choose can affect network latency for clients accessing your databases.  Requires the **Atlas region name**, see the reference list for [AWS](https://docs.atlas.mongodb.com/reference/amazon-aws/), [GCP](https://docs.atlas.mongodb.com/reference/google-gcp/), [Azure](https://docs.atlas.mongodb.com/reference/microsoft-azure/).
     Do not specify this field when creating a multi-region cluster using the replicationSpec document or a Global Cluster with the replicationSpecs array.
 * `provider_volume_type` - (AWS - Optional) The type of the volume. The possible values are: `STANDARD` and `PROVISIONED`.  `PROVISIONED` required if setting IOPS higher than the default instance IOPS.