Update manage-license-billing.md

Author: AbdullahMSFT

docs/sql-server/azure-arc/manage-license-billing.md

@@ -211,6 +211,10 @@ SQL Server licenses with Software Assurance or pay-as you go (`PAYG`) can benefi
 
 To help you manage the failover benefits and remain compliant, Azure Extension for SQL Server automatically detects the passive instances for availability groups (AGs) or failover clustered instances (FCIs) and reflects the use of the SQL Server software by emitting special $0 meters for disaster recovery, as long as you properly configured the LicenseType property.
 
+During the failovers, the extension is aware of the transition and automatically switches the ESU billing to the active replica without new bill-back charges and follow below logic 
+- If the period of failover <= 90 days, ESU billback occurs only from the prior watermark of usage upload
+- If the period of failover > 90 days, ESU billback occurs from the start of the ESU period. 
+
 ### To qualify as passive instance for an availability group (AG)
 
 - All replicas present in the operating system environment (OSE) must be secondary.