tag the terrain material diffuse tool label with more context

the name alone doesn't make it clear that that texture is stretched over the whole map and sampled to create the _basetex.dds files

Templates/BaseGame/game/tools/worldEditor/gui/guiTerrainMaterialDlg.ed.gui

@@ -195,7 +195,7 @@ $guiContent = new GuiControl(TerrainMaterialDlg,EditorGuiGroup) {
                tooltip = "Change the Active Diffuse Map for this layer";
             };
             new GuiTextCtrl() {
-               text = "Diffuse";
+               text = "Diffuse (Overlay)";
                position = "56 -3";
                extent = "39 18";
                profile = "EditorTextProfile";