Mk/bsd.options.desc.mk: add "PIPEWIRE_DESC?= PipeWire multimedia server support"

Keep custom description in ports:
graphics/spectacle
multimedia/obs-studio
net/krfb
x11-wm/gamescope

Approved by: arrowd (mentor)
Differential Revision: https://reviews.freebsd.org/D41038
MFH: 2023Q3