Update rediscloud_subscription.md (#326)

Updates the AZ selection field description

Author: noham14

docs/resources/rediscloud_subscription.md

@@ -117,10 +117,10 @@ The cloud_provider `region` block supports:
 * `networking_vpc_id` - (Optional) Either an existing VPC Id (already exists in the specific region) or create a new VPC
 (if no VPC is specified). VPC Identifier must be in a valid format (for example: ‘vpc-0125be68a4625884ad’) and existing
 within the hosting account.
-* `preferred_availability_zones` - (Required) Availability zones deployment preferences (for the selected provider & region).
+* `preferred_availability_zones` - (Required) Availability zones deployment preferences (for the selected provider & region). If multiple_availability_zones is set to 'true', you must select three availability zones from the list.
 
 ~> **Note:** The preferred_availability_zones parameter is required for Terraform, but is optional within the Redis Enterprise Cloud UI. 
-This difference in behaviour is to guarantee that a plan after an apply does not generate differences.  In AWS Redis internal cloud account, please set an empty list.
+This difference in behaviour is to guarantee that a plan after an apply does not generate differences. In AWS Redis internal cloud account, please set the zone IDs.
 
 ### Timeouts