Reword need for token

proposals/2778-appservice-login.md

@@ -6,8 +6,10 @@ do so, this proposal suggests implementing an appservice extension to the
 [`POST /login` endpoint](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-login).
 
 Appservice users do not usually need to login as they do not need their own access token, and do not
-traditionally need a "device". However E2E encryption demands that all users in a room have a device
-which means bridge users need to be able to generate a device on demand.
+traditionally need a "device". However, E2E encryption demands that at least one user in a room has a Matrix device
+which means bridge users need to be able to generate a device on demand. In the past, bridge developers
+have used the bridge bot's device for all bridge users in the room, but this causes problems should the bridge
+wish to only join ghosts to a room (e.g. for DMs).
 
 ## Proposal