Documentation for pa_*_drain does not specify one callback only

[boss_mc]

As it stands, repeated calls to pa_*_drain lead to only the last callback specified being called. This is all fine and good but the documentation does not make this clear and this can lead to deadlocks pretty easily.

It might be an idea to have a queueing system whereby each registered callback gets called in the order they were registered but this is not necessary.

[lennart]

Fixed in r2952f28c06b254f358ec9b3354dfc05c17a7b871