Add documentation to the docstring of the appropriate function
explaining how to disable the spesmilo#6960 test for dusting transactions

TODO: it may make sense to document passing this option on the command
line as well, since editing the config file by hand is a recipe for
trouble.

Unfortunately this is not as simple as doing:
```
electrum --unconf_utxo_freeze_threshold=0
```
and I can't find how electrum takes config arguments on the command
line. Maybe someone else knows?

- document that coins must be un-frozen using the Coins tab
- document using getconfig/setconfig as better than manually editing the
  config