doc: Add example of how to mix private and public keys in descriptors

It took me quite a while to understand how to use both xpriv and public keys in a single descriptor. The trick was to use xprv key + derivation path. Also, it was not obvious that Bitcoin Core would figure it out and sign a multisig transaction if the xpriv is present in the descriptor. As a newbie to Bitcoin Core, I had to consult stackexchange to understand that.

Adding an example would make it easier to "gotcha" for the reader.