fix: add proper description for `User` class

lib/src/user.dart

@@ -18,7 +18,16 @@
 
 import 'package:matrix/matrix.dart';
 
-/// Represents a Matrix User which may be a participant in a Matrix Room.
+/// Represents a user in the context of a Matrix room, not a global user profile.
+///
+/// This class extends [StrippedStateEvent] to handle room-specific user state,
+/// including membership status, display name, and avatar within that room.
+/// The user information is derived from room member state events.
+///
+/// For example, a user may have different display names or avatars in different rooms,
+/// and this class represents that room-specific view of the user rather than their
+/// global profile.
+///
 class User extends StrippedStateEvent {
   final Room room;
   final Map<String, Object?>? prevContent;