Update KQL queries section with query considerations

Added considerations regarding querying legacy tables, empty tables, workspace selection, cost implications, data retention, and performance of KQL queries against the data lake.

Author: zeinab-mk

articles/sentinel/datalake/kql-queries.md

@@ -133,6 +133,7 @@ external_table("AADRiskyUsers")
 ## Query considerations and limitations
 
 + Querying legacy tables such as AzureDiagnostics is not supported.
++ Empty tables don’t appear in schema view, and queries aren’t supported until the table contains data.
 + Queries are run against the workspaces you selected. Make sure you select the correct workspaces before running a query.
 + Executing KQL queries on the Microsoft Sentinel data lake incurs charges based on query billing meters. For more information, see [Plan costs and understand Microsoft Sentinel pricing and billing](../billing.md#data-lake-tier).
 + Review data ingestion and table retention policy. Before setting query time range, be aware of data retention on your data lake tables and whether data is available for selected time range. For more information, see [Manage data tiers and retention in Microsoft Defender portal](https://aka.ms/manage-data-defender-portal-overview).