Update capacity-reservation-overview.md

Added description for pinned

Author: tferdou1

articles/virtual-machines/capacity-reservation-overview.md

@@ -90,6 +90,7 @@ From this example accumulation of Minutes Not Available, here's the calculation
     - UltraSSD storage
     - VMs resuming from hibernation 
     - VMs requiring vnet encryption
+- Pinned subscription cannot use the feature
 - Only the subscription that created the reservation can use it. 
 - Reservations are only available to paid Azure customers. Sponsored accounts such as Free Trial and Azure for Students aren't eligible to use this feature.