mlxsw: spectrum_qdisc: Clarify a comment

Expand the comment at mlxsw_sp_qdisc_prio_graft() to make the problem that
this function is trying to handle clearer.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>