relative-pointer: Mention interaction with `wl_pointer.frame`

XWayland assumes relative motion events are followed by a frame if wl_pointer has version 5 or greater: https://gitlab.freedesktop.org/xorg/xserver/-/blob/master/hw/xwayland/xwayland-input.c

But the protocol didn't mention frame. This may be implied, but should be addressed specifically.

This should match existing implementations.