sound: Update comment and channel insertion in vchan_create()

The comment and rationale behind choosing CHN_INSERT_SORT_DESCEND()
instead of CHN_INSERT_SORT_ASCEND() are no longer relevant as of FILLME
("sound: Allocate vchans on-demand").

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48962

(cherry picked from commit 2868776c9c6f5313a5676786f642b766f103d780)