Update modern-control-combobox.md

Author: fafuxa-ms

powerapps-docs/maker/canvas-apps/controls/modern-controls/modern-control-combobox.md

@@ -20,6 +20,8 @@ contributors:
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 # Combobox modern control in Power Apps (preview)
 
+[This article is pre-release document and is subject to change.]
+
 A control that allows users to make selections from provided choices and supports search and multiple selections. 
 
 ## Description