Add limitation for AGs on FCI.

Author: MikeRayMSFT

docs/sql-server/azure-arc/manage-availability-group.md

@@ -59,6 +59,7 @@ This will initiate a planned, manual failover on the AG replica.
 ### Considerations
 
 - Currently, manual planned failover is the only mode of failover supported
+- Azure Arc-enabled SQL Server features are not supported for availability groups on failover cluster instances.
 
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