proto: correct the ei_device.interface documentation

Remove leftovers from when this was an ei_device.capability event
but since removed, see [1]. Instead make clear that this needs to be
one of the bound interfaces.

[1] a902d5dbd8 ("protocol: replace the capabilities enum with an interface list")

proto/protocol.xml

@@ -766,22 +766,19 @@
       <description summary="device capability notification">
         Notification that a new device has a sub-interface.
 
-        This event may be sent for the
-        - "ei_pointer" interface if the device has the
-          ei_device.capabilities.pointer capability
-        - "ei_pointer_absolute" interface if the device has the
-          ei_device.capabilities.pointer_absolute capability
-        - "ei_scroll" interface if the device has the
-          ei_device.capabilities.scroll capability
-        - "ei_button" interface if the device has the
-          ei_device.capabilities.button capability
-        - "ei_keyboard" interface if the device has the
-          ei_device.capabilities.keyboard capability
-        - "ei_touchscreen" interface if the device has the
-          ei_device.capabilities.touchscreen capability
+        This event may be sent for the following interfaces:
+        - "ei_pointer"
+        - "ei_pointer_absolute"
+        - "ei_scroll"
+        - "ei_button"
+        - "ei_keyboard"
+        - "ei_touchscreen"
         The interface version is equal or less to the client-supported
         version in ei_handshake.interface_version for the respective interface.
 
+        It is a protocol violation to send a notification for an interface that
+        the client has not bound to with ei_seat.bind.
+
         This event is optional and sent immediately after object creation
         and at most once per interface.
         It is a protocol violation to send this event after the ei_device.done event.