sound: Clarify userland/vchan relationship in sndstat feeder chain

[christos]

Summary

When vchans are enabled, the primary channels do not interact with
userland, but with the vchans.

With vchans enabled:

$ sndctl feederchain
dsp0.play.0.feederchain=[vchans -> [...] -> hardware]
dsp0.record.0.feederchain=[hardware -> [...] -> vchans]

With vchans disabled:

dsp0.play.0.feederchain=[userland -> [...] -> hardware]
dsp0.record.0.feederchain=[hardware -> [...] -> userland]

Sponsored by: The FreeBSD Foundation
MFC after: 1 week