Consul Docs: Azure auto-join MSI clarification (#1346)

* Add note about MSI and client IDs
* Style suggestions from @boruszak

Co-authored-by: Jeff Boruszak <[email protected]>

Author: nphilbrook

content/consul/v1.22.x/content/docs/deploy/server/cloud-auto-join.mdx

@@ -191,6 +191,8 @@ Variables can also be provided by environmental variables:
 - `ARM_CLIENT_ID` for client
 - `ARM_CLIENT_SECRET` for secret access key
 
+If you omit any of the `TENANT`, `CLIENT`, or `CLIENT_SECRET` arguments, then Consul defaults to Managed Service Identities instead. Any client identification that was specified is ignored for the purposes of MSI. To pass client identity to MSI, you must use the `AZURE_CLIENT_ID` environment variable. For more information, refer to the [Azure autorest package documentation](https://github.com/Azure/go-autorest/blob/autorest/v0.11.29/autorest/azure/auth/auth.go#L43).
+
 Use these configuration parameters when using tags:
 
 - `tag_name` - the name of the tag to auto-join on.