[DeckShare] Restore the banner-text doc comment

Re-add the doc block above refreshBannerCardText() that was removed as part of
the share-selection work; it documents the coupling to refreshBannerCardToolTip.

cockatrice/src/interface/widgets/visual_deck_storage/deck_preview/deck_preview_widget.cpp

@@ -295,6 +295,10 @@ void DeckPreviewWidget::updateTagsVisibility(bool visible)
     }
 }
 
+/**
+ * Refreshes the banner card text.
+ * This also calls `refreshBannerCardToolTip`, since those two often need to be updated together.
+ */
 void DeckPreviewWidget::refreshBannerCardText()
 {
     bannerCardDisplayWidget->setOverlayText(getDisplayName());