snd_uaudio.4: Rework SYNOPSIS, add HARDWARE, SPDX

+ Rework synopsis for consistency, adding sysctls
+ Add a HARDWARE section for HW relnotes
+ tag SPDX

MFC after: 3 days
Reviewed by: christos, pauamma
Differential Revision: https://reviews.freebsd.org/D51240

(cherry picked from commit 701072154af0881ef395736a06d09ffa47cfc096)