add: 'The PTR records are those for the computer accounts created by the storage in Active Directory'

Author: b-hchen

articles/azure-netapp-files/understand-guidelines-active-directory-domain-service-site.md

@@ -72,7 +72,7 @@ Ensure that you meet the following requirements about the DNS configurations:
 * Ensure that [the SRV records registered by the AD DS Net Logon service](https://social.technet.microsoft.com/wiki/contents/articles/7608.srv-records-registered-by-net-logon.aspx) have been created on the DNS servers.
 * Ensure that the PTR records for the AD DS domain controllers used by Azure NetApp Files have been created on the DNS servers.
 * Azure NetApp Files supports standard and secure dynamic DNS updates. If you require secure dynamic DNS updates, ensure that secure updates are configured on the DNS servers.
-* If dynamic DNS updates are not used, you need to manually create A record and PTR records for Azure NetApp Files SMB volumes.
+* If dynamic DNS updates are not used, you need to manually create A record and PTR records for Azure NetApp Files SMB volumes. The PTR records are those for the computer accounts created by the storage in Active Directory. 
 * For complex or large AD DS topologies, [DNS Policies or DNS subnet prioritization may be required to support LDAP enabled NFS volumes](#ad-ds-ldap-discover).  
 
 ### Time source requirements