mixer.3: Fix mixer_get_mode() description

Improve wording and also fix the constants' names.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D46220

(cherry picked from commit 6e744de1a3dc5dde8d2ee51e97a1224a01bdfb21)