Update Add-Computer.md (#11841)

* Update Add-Computer.md

specify DNS FQDN is required. it is a regression, but really the change is makes is good...

* Update Add-Computer.md with domain join hardening details

---------

Co-authored-by: Sean Wheeler <sean.wheeler@microsoft.com>

Author: HerbertMauerer

reference/5.1/Microsoft.PowerShell.Management/Add-Computer.md

@@ -454,7 +454,11 @@ Accept wildcard characters: False
 ### -Server
 
 Specifies the name of a domain controller that adds the computer to the domain. Enter the name in
-DomainName\ComputerName format. By default, no domain controller is specified.
+FQDN format. By default, no domain controller is specified.
+
+Beginning in August 2024, security hardening for domain join requires that you use the FQDN of the
+domain controller. For example: `DC1.contoso.com`. For more information, see
+[NetJoin Domain Join Hardning Changes](https://support.microsoft.com/topic/kb5020276-netjoin-domain-join-hardening-changes-2b65a0f3-1f4c-42ef-ac0f-1caaf421baf8).
 
 ```yaml
 Type: System.String