tinyformat: Add doc to Bitcoin Core specific strprintf
tinyformat: Add doc to Bitcoin Core specific strprintf

Former-commit-id: 0d535d1f51c349be4acd7b46b8026465f2d8f956
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin