Update alter-merge-mirroring-policy-command.md

Author: anshulsharmas

data-explorer/kusto/management/alter-merge-mirroring-policy-command.md

@@ -41,6 +41,8 @@ Changes the tables's [mirroring policy](mirroring-policy.md). The mirroring poli
 |--|--|--|
 |`IsEnabled`| `bool`| A Boolean value that determines whether the mirroring policy is enabled. Default is `true`. When the mirroring policy is disabled and set to `false`, the underlying mirroring data is soft-deleted and retained in the database. |
 |`TargetLatencyInMinutes`| `int`| The write operation delay in minites. By default, the write operation can take up to 3 hours or until there's 256 MB of data available. You can adjust the delay to a value between 5 minutes and 3 hours. |
+|`backfill`| `bool`| If set to `true`, mirroring will start from `effectiveDateTime` if provided, or from the beginning of the table otherwise. Default is `false`.|
+|`effectiveDateTime`| `datetime`| Relevant only when you're using backfill. If provided, mirroring will start from the specified point in time. backfill must also be set to true. |
 
 [!INCLUDE [mirroring-note](../includes/mirroring-note.md)]
 
@@ -55,7 +57,7 @@ In the following example, a table called *myTable* is mirrored. The data is part
   partition by (Name: string=Name, Date: datetime= startofday(timestamp))
   dataformat=parquet
   with
-  (IsEnabled=true)
+  (IsEnabled=true, backfill=true)
 ```
 
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