update the BCP420 issue description

Author: mumian

articles/azure-resource-manager/bicep/diagnostics/bcp420.md

@@ -20,7 +20,7 @@ Error
 
 ## Examples
 
-The following code triggers BCP420 because the scope can't be determined at the compile time.
+The following code triggers BCP420 because the scope property uses a conditional expression that depends on the runtime value of the **targetResourceGroupName** parameter. Bicep requires that scope expressions resolve to a specific scope at compile time, without relying on parameter values or dynamic logic. Since the compiler can't determine the scope without evaluating **targetResourceGroupName**, it throws BCP420.
 
 ```bicep
 param targetResourceGroupName string = 'my-target-rg'