MSC1840: Don't specify an explicit default for untyped rooms

Initially I was considering the default to be `m.messaging`, but
@julianfoad brings up a good point that this isn't necessarily a good
assumption. I'm inclined to leave the decision of what to do with
untyped rooms up to the clients, while recommending that they set room
types for new rooms going forward.

proposals/1840-room-types.md

@@ -40,10 +40,12 @@ rules for textual identifiers.
 
 This proposal defines `m.messaging` as the (thus far) traditional
 instant-messaging style room found in Element and most other Matrix
-clients today. A room without an `m.room.type` state event may be
-considered to be of type `m.messaging` for backwards-compatibility
-reasons, but IM clients aware of room types should add an
-`m.messaging` room type state event when creating a new room.
+clients today. Client developers may handle rooms without a type in a
+way that makes sense for each client; for example, it is expected that
+an IM client would list both untyped rooms (for backwards
+compatibility) and rooms of type `m.messaging` in the room
+list. However, clients that are aware of room types should add the
+relevant room type state event when creating new rooms.
 
 This by itself could be used by a client to properly display rooms
 based on their intended usage. However, to optimize the use of