Clarify when clients MUST display as from sender

Clarified that clients must display the message as from the sender if
the sender is in the user's deny list (even if the sender is in the
allow list as well).

Clarified the text to be a lot more readable.

proposals/3464-allow-users-to-post-on-behalf-of-users.md

@@ -113,21 +113,16 @@ of those lists.
 
 #### Timeline display
 
-If the message content does not have an `m.on_behalf_of` field, then clients
-MUST display the message as if it came from the sender.
-
-If the message content has an `m.on_behalf_of` field, but the user listed in
-that field is not in the room, then clients MUST display the message as if it
-came from the sender.
-
-If the message content has an `m.on_behalf_of` field and the message sender is
-in in neither the `allow` nor the `deny` list for the user listed in
-`m.on_behalf_of`, then clients MUST display the message as if it came from the
-sender.
-
-If the message content has an `m.on_behalf_of` field and the message sender is
-in the `deny` list for the user listed in `m.on_behalf_of`, then clients MUST
-display the message as if it came from the sender.
+Clients MUST display the message as if it came from the sender in any of the
+following cases:
+
+* The message content does not have an `m.on_behalf_of` field.
+* The message content has an `m.on_behalf_of` field, but the user listed in that
+  field is not in the room.
+* The message content has an `m.on_behalf_of` field, but the sender is in the
+  `deny` list for the user listed in `m.on_behalf_of`.
+* The message content has an `m.on_behalf_of` field, but the sender is not in
+  the `allow` list for the user listed in `m.on_behalf_of`.
 
 If the message content has an `m.on_behalf_of` field and the message sender is
 in the `allow` list for the user listed in `m.on_behalf_of`, then