DBus: thighten the definition of the 'Id' for a device
We can't guarantee it's a sysname even though that is what we use for evdev
devices. Future devices may just be some custom-assigned name for a remote
dbus API
@@ -60,7 +60,9 @@ known to ratbagd.
- type: `s`, read-only, constant
An ID describing this device. This ID should not be used for presentation to
-the user.
+the user. This ID is unique for the device for the lifetime of the device
+but may be recycled when the device is removed. No guarantee is given for
+the content of the ID, the client should treat it as an opaque string.
#### `Description`
- type: `s`, read-only, constant
