Update known limitations for NAT Gateway

Clarified limitations regarding Standard SKU NAT Gateway and IPv6 support.

Author: aimee-littleton

articles/nat-gateway/nat-sku.md

@@ -64,7 +64,9 @@ StandardV2 NAT Gateway supports flow logs through Azure Monitor. Flow logs provi
 
 * Requires StandardV2 SKU Public IP addresses and prefixes. Standard SKU public IPs aren’t supported. 
 
-* Standard SKU NAT Gateway can’t be upgraded to StandardV2 SKU NAT Gateway. You must deploy StandardV2 SKU NAT Gateway and replace Standard SKU NAT Gateway. 
+* Standard SKU NAT Gateway can’t be upgraded to StandardV2 SKU NAT Gateway. You must deploy StandardV2 SKU NAT Gateway and replace Standard SKU NAT Gateway.
+  
+* Terraform doesn't yet support attachment of IPv6 StandardV2 public IPs to StandardV2 NAT Gateway. IPv4 StandardV2 public IPs can be attached to StandardV2 NAT Gateway with terraform. No other clients are impacted.
 
 * Custom IP prefixes (BYOIP public IPs) aren't supported with StandardV2 NAT Gateway. Only StandardV2 SKU Azure public IPs are supported.