guidance on diff between clusters and advanced_clusters (#822)

Add guidance on diff between clusters and advanced_clusters till we get to INTMDB-338

Author: themantissa

website/docs/r/advanced_cluster.html.markdown

@@ -12,6 +12,9 @@ description: |-
 
 More information on considerations for using advanced clusters please see [Considerations](https://docs.atlas.mongodb.com/reference/api/cluster-advanced/create-one-cluster-advanced/#considerations)
 
+~> **IMPORTANT:**
+<br> &#8226; The primary difference between [`mongodbatlas_cluster`](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/cluster) and [`mongodbatlas_advanced_cluster`](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/advanced_cluster) is that `mongodbatlas_advanced_cluster` supports multi-cloud clusters.  We recommend new users start with the `mongodbatlas_advanced_cluster` resource.  
+
 -> **NOTE:** Groups and projects are synonymous terms. You may find group_id in the official documentation.
 
 -> **NOTE:** A network container is created for a advanced cluster to reside in if one does not yet exist in the project.  To  use this automatically created container with another resource, such as peering, the `container_id` is exported after creation.

website/docs/r/cluster.html.markdown

@@ -15,7 +15,8 @@ description: |-
 -> **NOTE:** A network container is created for a cluster to reside in if one does not yet exist in the project.  To  use this automatically created container with another resource, such as peering, the `container_id` is exported after creation.
 
 ~> **IMPORTANT:**
-<br> &#8226; Free tier cluster creation (M0) is  supported.
+<br> &#8226; New Users: If you are not already using `mongodbatlas_cluster` for your deployment we recommend starting with the [`mongodbatlas_advanced_cluster`](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/advanced_cluster).  `mongodbatlas_advanced_cluster` has all the same functionality as `mongodbatlas_cluster` but also supports multi-cloud clusters.  
+<br> &#8226; Free tier cluster creation (M0) is supported.
 <br> &#8226; Shared tier clusters (M0, M2, M5) cannot be upgraded to higher tiers via API or by this Provider.  WARNING! If you attempt to upgrade from an existing shared tier cluster that you manage with this Provider to a dedicated cluster (M10+) Terraform will see it as a request to destroy the shared tier cluster and as a request to create a dedicated tier cluster, i.e. Terraform will not see it as a request to upgrade.   If you accept the plan in this case the shared tier cluster would be destroyed and you would lose the data on that cluster. Do not attempt to upgrade from the shared to dedicated tier via this Provider, it is not supported!  
 <br> &#8226; Changes to cluster configurations can affect costs. Before making changes, please see [Billing](https://docs.atlas.mongodb.com/billing/).   
 <br> &#8226; If your Atlas project contains a custom role that uses actions introduced in a specific MongoDB version, you cannot create a cluster with a MongoDB version less than that version unless you delete the custom role.