[Doc] Update documentation for importing some MWS resources (#4281)

## Changes
Some resources are documented as not being importable, but some actually
are. This PR adds docs to a few that I was able to test out myself.

## Tests
Imported existing resources into a local TF project, which worked.

Author: mgyucht

docs/resources/mws_customer_managed_keys.md

@@ -253,7 +253,15 @@ In addition to all arguments above, the following attributes are exported:
 
 ## Import
 
-!> Importing this resource is not currently supported.
+This resource can be imported by Databricks account ID and customer managed key ID.
+
+```sh
+terraform import databricks_mws_customer_managed_keys.this '<account_id>/<customer_managed_key_id>'
+```
+
+~> This resource does not support updates. If your configuration does not match the existing resource,
+   the next `terraform apply` will cause the resource to be destroyed and recreated. After importing,
+   verify that the configuration matches the existing resource by running `terraform plan`.
 
 ## Related Resources
 

docs/resources/mws_networks.md

@@ -215,7 +215,15 @@ In addition to all arguments above, the following attributes are exported:
 
 ## Import
 
-!> Importing this resource is not currently supported.
+This resource can be imported by Databricks account ID and network ID.
+
+```sh
+terraform import databricks_mws_networks.this '<account_id>/<network_id>'
+```
+
+~> This resource does not support updates. If your configuration does not match the existing resource,
+   the next `terraform apply` will cause the resource to be destroyed and recreated. After importing,
+   verify that the configuration matches the existing resource by running `terraform plan`.
 
 ## Related Resources
 

docs/resources/mws_private_access_settings.md

@@ -85,7 +85,11 @@ In addition to all arguments above, the following attributes are exported:
 
 ## Import
 
-!> Importing this resource is not currently supported.
+This resource can be imported by Databricks account ID and private access settings ID.
+
+```sh
+terraform import databricks_mws_private_access_settings.this '<account_id>/<private_access_settings_id>'
+```
 
 ## Related Resources
 

docs/resources/mws_storage_configurations.md

@@ -55,7 +55,15 @@ In addition to all arguments above, the following attributes are exported:
 
 ## Import
 
-!> Importing this resource is not currently supported.
+This resource can be imported by Databricks account ID and storage configuration ID.
+
+```sh
+terraform import databricks_mws_storage_configurations.this '<account_id>/<storage_configuration_id>'
+```
+
+~> This resource does not support updates. If your configuration does not match the existing resource,
+   the next `terraform apply` will cause the resource to be destroyed and recreated. After importing,
+   verify that the configuration matches the existing resource by running `terraform plan`.
 
 ## Related Resources
 

docs/resources/mws_workspaces.md

@@ -392,7 +392,18 @@ You can reset local DNS caches before provisioning new workspaces with one of th
 
 ## Import
 
-!> Importing this resource is not currently supported.
+This resource can be imported by Databricks account ID and workspace ID.
+
+```sh
+terraform import databricks_mws_networks.this '<account_id>/<workspace_id>'
+```
+
+~> Not all fields of `databricks_mws_workspaces` can be updated without causing the workspace to be recreated.
+   If the configuration for these immutable fields does not match the existing workspace, the workspace will
+   be deleted and recreated in the next `terraform apply`. After importing, verify that the configuration
+   matches the existing resource by running `terraform plan`. The only fields that can be updated are
+   `credentials_id`, `network_id`, `storage_customer_managed_key_id`, `private_access_settings_id`,
+   `managed_services_customer_managed_key_id`, and `custom_tags`.
 
 ## Related Resources