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>