update msat on Pay response

[lvaccaro]

Update satoshi returned values of pay command to the doc https://github.com/ElementsProject/lightning/blob/master/doc/lightning-pay.7.md#return-value

[vincenzopalazzo]

Thanks you will find some others down the road. I should implement the code generator. You can use also the call that allows you to pass any struct.

BTW I will merge if you move the commit header to fix(rpc): update msat on Pay response. In this way I can generate the changelog :)

[vincenzopalazzo]

ACK 5033929