Fix docs for some responses

include/mtx/responses/device.hpp

@@ -18,6 +18,7 @@
 namespace mtx {
 namespace responses {
 
+//! Information about one of your devices.
 struct Device
 {
     //! **Required.** Identifier of this device.

include/mtx/responses/empty.hpp

@@ -14,14 +14,17 @@
 namespace mtx {
 namespace responses {
 
-// Used to represent empty responses
+//! Used to represent empty responses
 struct Empty
 {
     friend void from_json(const nlohmann::json &, Empty &);
 };
 
+//! The response for a logout.
 using Logout      = Empty;
+//! The response after changing your display name.
 using DisplayName = Empty;
+//! The response after inviting a user to a room.
 using RoomInvite  = Empty;
 }
 }