Update azure-monitor-agent-transformation.md

Author: JeffreyWolford

articles/azure-monitor/agents/azure-monitor-agent-transformation.md

@@ -39,7 +39,11 @@ To complete this procedure, you need:
     1. Run a basic query the custom logs table to view table data.
 1. Use the query window to write and test a query that transforms the raw data in your table.
 
-    For information about the KQL operators that transformations support, see [Structure of transformation in Azure Monitor](../essentials/data-collection-transformations-structure.md#kql-limitations). 
+    For information about the KQL operators that transformations support, see [Structure of transformation in Azure Monitor](../essentials/data-collection-transformations-structure.md#kql-limitations).
+   
+   > [!Note]
+   > The only columns that are available to apply transfroms against are TimeGenerated and RawData.  Other columns are added to the table automatically after the transformation and are not availiable at the time of transformation.
+   > The _ResourceId column can't be used in the trasnformation.
 
     **Example**
 
@@ -105,4 +109,4 @@ Learn more about:
 - [Data collection rules](../essentials/data-collection-rule-overview.md). 
 - [Data collection endpoints](../essentials/data-collection-endpoint-overview.md). 
 
- 
+