[Doc] Add warning about disabling legacy features and default catalog (#4905)

## Changes
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## Tests
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- [x] relevant change in `docs/` folder
- [x] has entry in `NEXT_CHANGELOG.md` file

Author: alexott

NEXT_CHANGELOG.md

@@ -34,6 +34,7 @@
 * Improve docs for `databricks_compliance_security_profile_setting` ([#4880](https://github.com/databricks/terraform-provider-databricks/pull/4880)).
 * Improve instructions for the Terraform Exporter ([#4892](https://github.com/databricks/terraform-provider-databricks/pull/4892)).
 * Improve documentation for service principal data sources ([#4900](https://github.com/databricks/terraform-provider-databricks/pull/4900)).
+* Add warning about disabling legacy features and default catalog ([#4905](https://github.com/databricks/terraform-provider-databricks/pull/4905)).
 * Improve documentation for grants resources ([#4906](https://github.com/databricks/terraform-provider-databricks/pull/4906))
 
 ### Exporter

docs/resources/disable_legacy_features_setting.md

@@ -8,7 +8,10 @@ subcategory: "Settings"
 
 The `databricks_disable_legacy_features_setting` resource allows you to disable legacy features on newly created workspaces.
 
+~> Before disabling legacy features, make sure that default catalog for the workspace is set to value different than `hive_metastore`!  You can set it using the [databricks_default_namespace_setting](default_namespace_setting.md) resource.
+
 When this setting is on, the following applies to new workspaces:
+
 - Disables the use of DBFS root and mounts.
 - Hive Metastore will not be provisioned.
 - Disables the use of 'No-isolation clusters'.
@@ -17,6 +20,14 @@ When this setting is on, the following applies to new workspaces:
 ## Example Usage
 
 ```hcl
+# Change default catalog to anything than `hive_metastore`
+resource "databricks_default_namespace_setting" "this" {
+  namespace {
+    value = "default_catalog"
+  }
+}
+
+# Disable legacy features
 resource "databricks_disable_legacy_features_setting" "this" {
   disable_legacy_features {
     value = true