Document rt priority value which is hardcoded

[MaxPayne86]

Let's make an example, on Zynthian we have jackd running with rt priority 70 (-P 70). I know then that jackd creates a thread with higher priority, usually 75. Making these assumptions, it is correct to have a default value for rtprio = 80? I don't know the configuration in Mod Duo, but I think similar. I can submit a patch that accepts also rt prio argument so it can works with more systems...any clue?

@falkTX @jofemodo