Document lack of DiffDisk support in instance mix

Added notes to the FAQ and overview documentation clarifying that instance mix does not currently support `diffDiskSettings` on the OS disk. This improves clarity for users configuring VM scale sets with instance mix.

Author: brittanyrowe

articles/virtual-machine-scale-sets/instance-mix-faq-troubleshooting.md

@@ -34,6 +34,9 @@ Yes, you can apply your reserved instance prices and savings plan with instance
 ### Can I use an on-demand capacity reservation with instance mix?
 No, you can't use an on-demand capacity reservation with instance mix.
 
+### Can I use DiffDisk settings with instance mix?
+No, instance mix currently doesn't support `diffDiskSettings` on the OS disk.
+
 ## Troubleshooting
 | Error Code                                 | Error Message                                                                                                        | Troubleshooting options                                                                                                                                                                                                                                                                                              |
 | ------------------------------------------ | -------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |

articles/virtual-machine-scale-sets/instance-mix-overview.md

@@ -110,6 +110,7 @@ Before you deploy an instance mix scale set:
 - VMs with different storage interfaces (SCSI vs NVMe) can't be mixed.
 - You can't mix VM SKUs that use premium storage and non-premium storage in the same instance mix.
 - All VMs must share the same Security Profile and local disk configuration.
+- **DiffDisk settings**: Instance mix currently doesn't support `diffDiskSettings` on the OS disk.
 - Instance mix doesn't support: Standby Pools, Azure Dedicated Host, Proximity Placement Groups, or on-demand capacity reservations.
 
 ## Next steps