feat(rdb): add docs and split dhcp test

Author: Monitob

docs/resources/rdb_instance.md

@@ -160,11 +160,11 @@ In addition to all arguments above, the following attributes are exported:
     - `name` - Name of the endpoint.
     - `hostname` - Name of the endpoint.
 - `private_network` - List of private networks endpoints of the database instance.
-  - `endpoint_id` - The ID of the endpoint of the private network.
-  - `ip` - IP of the endpoint.
-  - `port` - Port of the endpoint.
-  - `name` - Name of the endpoint.
-  - `hostname` - Name of the endpoint.
+    - `endpoint_id` - The ID of the endpoint of the private network.
+    - `ip` - IP of the endpoint.
+    - `port` - Port of the endpoint.
+    - `name` - Name of the endpoint.
+    - `hostname` - Name of the endpoint.
 - `certificate` - Certificate of the database instance.
 - `organization_id` - The organization ID the Database Instance is associated with.