docs: clarify description key is optional for composite actions

Mark description field as optional for composite actions and add recommendation note.

Author: Xiyu-z

content/actions/reference/workflows-and-actions/metadata-syntax.md

@@ -33,7 +33,8 @@ category:
 
 ## `description`
 
-**Required** A short description of the action.
+**Optional** A short description of the action. While optional, it is highly recommended to include a description.
+
 
 ## `inputs`