doc: how to view internal info about a coin (tx output)

This (`GetTxOut`) will probably come in handing debugging soon:

```sh
dash-cli -conf="${HOME}/.dashmate/local_seed/core/dash.conf" \
    --rpcwallet=luke \
    gettxout 9d114ea97684fdae47946f733aa2d9d3d4b21df16f5b0092c44e59cc503ed188 0
```