Documentation improvement: Coin selection

[BTCBellyButton]

Hi all, It could be helpful to add a short paragraph to this one: https://github.com/JoinMarket-Org/joinmarket-clientserver/blob/master/docs/USAGE.md#try-out-a-coinjoin-using-sendpaymentpy and explain how to select Coins/UTXOs to sweep.

From that section one can be led to think that it's only possible to sweep all the coins in a mixed depth merging them in a single destination. But how to sweep just one of the UTXOs (like an expired Fidelity Bond for example) isn't immediately clear.

A brief explanation that in that case you need to freeze all the UTXOs of that particular Mixed Depth and unfreeze only the one you want to sweep would be helpful!

From real life :-) :