Note required levels of descendant transactions in get_spendable_outputs

Note required levels of descendant transactions in get_spendable_outputs

Three levels of descendant transactions starting from the channel's
funding transaction should cover all potential spendable outputs.

The first level covers the commitment transaction.

The second level covers the to_self claims, to_remote claims,
second-stage HTLC claims and justice transactions.

The third levels covers the justice transactions on second-stage HTLCs,
and to_self claims on second-stage HTLCs.