protocol: Clarify how clients can cancel a drag operation
Explicitly say that destroying a wl_data_source previously used in a wl_data_device::start_drag request will cancel the DND session. This is currently the only way to do it from client side (besides those "indirect" ones already documented) and all compositors work like that but it is not clear spec-wise it is the expected behaviour.
Signed-off-by: Nick Diego Yamane nickdiego@igalia.com