doc(ui): Explain why m.room.create is necessary.

This patch explains why having `m.room.create` is necessary. It's not
only about the room previews, but also about the room versions, and thus
the tombstoned rooms.

crates/matrix-sdk-ui/src/room_list_service/mod.rs

@@ -90,6 +90,8 @@ const DEFAULT_REQUIRED_STATE: &[(StateEventType, &str)] = &[
     (StateEventType::RoomJoinRules, ""),
     (StateEventType::RoomTombstone, ""),
     // Those two events are required to properly compute room previews.
+    // `StateEventType::RoomCreate` is also necessary to compute the room
+    // version, and thus handling the tombstoned room correctly.
     (StateEventType::RoomCreate, ""),
     (StateEventType::RoomHistoryVisibility, ""),
     // Required to correctly calculate the room display name.