docs(embed): say why the Filter button is hidden in va mode

docs/embed-va.md

@@ -28,6 +28,8 @@ The `?embed=` parameter accepts three values:
 
 Chrome means the header, the back button and the sub-nav.  In `va` mode the editor is also VA-aware: the report's filters are captured and applied as you change selections, and the columns the grid shows can be controlled from the report's Edit Report interface.  Because the report owns the filter in that mode, Data Controller's own **Filter** button is not offered - use the report's controls instead.
 
+The two are not combined.  A filter built inside Data Controller and a filter driven by the report are alternatives, not layers: applying one replaces the other, and in `va` mode the report's filter is re-applied the next time the selection changes.  Showing the button there would offer a filter that silently stops applying as soon as the report is touched, which is why it is hidden rather than disabled.  If you need Data Controller's own filtering, embed with `embed=true` (or with no `embed` parameter) and leave the report's parameters unassigned.
+
 ## How VA drives the editor
 
 VA pushes a JSON payload to the iframe via `window.postMessage` whenever the selected rows change.  Data Controller listens for these messages and: