[Schema] Update version description

Make description for version attribute a little more clear about being required for toplevel cards.

Fixes #2958

schemas/adaptive-card.json

@@ -12,7 +12,7 @@
 	"properties": {
 		"version": {
 			"type": "string",
-			"description": "Schema version that this card requires. If a client is **lower** than this version, the `fallbackText` will be rendered. NOTE: Version is not required for cards within an `Action.ShowCard`",
+			"description": "Schema version that this card requires. If a client is **lower** than this version, the `fallbackText` will be rendered. NOTE: Version is not required for cards within an `Action.ShowCard`. However, it *is* required for the top-level card.",
 			"examples": [
 				"1.0", "1.1", "1.2"
 			]