Add documentation about qml z fix

Co-Authored-By: nallath <nallath@gmail.com>

plugins/PrepareStage/PrepareMenu.qml

@@ -52,7 +52,7 @@ Item
                 Cura.MachineSelector
                 {
                     id: machineSelection
-                    z: openFileButtonBackground.z - 1
+                    z: openFileButtonBackground.z - 1 //Ensure that the tooltip of the open file button stays above the item row.
                     Layout.minimumWidth: UM.Theme.getSize("machine_selector_widget").width
                     Layout.maximumWidth: UM.Theme.getSize("machine_selector_widget").width
                     Layout.fillWidth: true
@@ -117,4 +117,4 @@ Item
             }
         }
     }
-}
+}