Update control-flow-append-variable-activity.md

Fixes https://github.com/MicrosoftDocs/azure-docs/issues/100631
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cc: @chez-charlie

Author: jonburchel

articles/data-factory/control-flow-append-variable-activity.md

@@ -31,6 +31,9 @@ To use a Append Variable activity in a pipeline, complete the following steps:
 
    :::image type="content" source="media/control-flow-append-variable-activity/append-variable.png" alt-text="Shows the UI for an Append Variable activity.":::
 
+> [!NOTE]
+> The appended variable value does not appear in debug output unless you use a [Set Variable activity](control-flow-set-variable-activity.md) to explicitly set a new variable with its value.
+
 ## Type properties
 
 Property | Description | Required