sound: Clarify userland/vchan relationship in sndstat feeder chain

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:

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

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53504

(cherry picked from commit 59d7865d3c20e86b3c42096f4a8c06a5e2eebd29)