CLEANUP: stream-int: retro-document si_cs_io_cb()

CLEANUP: stream-int: retro-document si_cs_io_cb()

It took me 17 minutes this morning to figure where si->wait_event was
set (it's in si_reset() which should now probably be renamed since it
doesn't just perform a reset anymore but also an allocation) and what
its task was assigned to (si_cs_io_cb() even for applets and empty SI).

This is too confusing and not intuitive enough, let's at least add a
few comments for now to help figure how this stuff works next time.