[polaris.shopify.com] Update LegacyCard description

Co-authored-by: aveline <aveline@users.noreply.github.com>

Author: laurkim

polaris.shopify.com/content/components/legacy-card.md

@@ -1,6 +1,6 @@
 ---
 title: LegacyCard
-description: Cards are used to group similar concepts and tasks together to make Shopify easier for merchants to scan, read, and get things done.
+description: This is the legacy version of the Card component which is in the process of being updated to a more flexible and composable API. Cards are used to group similar concepts and tasks together to make Shopify easier for merchants to scan, read, and get things done.
 category: Structure
 keywords:
   - layout