Clarify BTC wallet export format
Currently, the output of `export-bitcoin-wallet` looks something like this:
```
{"descriptor":"wpkh(xprv9s21gel82lxs8nvu49v2839189932ddkjdf2j6Ymmk/84'/0'/0'/0/*)","blockheight":31337,"label":"asb-bitcoin"}
```
As it stands this cannot be imported into Electrum directly.

I tried converting it to a zprv using https://github.com/AbdussamadA/electrum-xprv however that didn't work either.

The documentation for `export-bitcoin-wallet` should explain how to convert the wallet descriptor into a format that works with standard wallets (like Electrum). Alternately, there should be an option to export the wallet in a compatible format.