x11-wm/sway: suggest mac_priority(4) after b22b6eeffd22

FreeBSD lacks setcap(8) + CAP_SYS_NICE, so permissions to set realtime
priorities like SCHED_RR are group-wide and disabled by default.

$ sway
[...]
00:00:00.024 [INFO] [sway/realtime.c:35] Failed to set scheduling priority to 0
[...]

https://github.com/swaywm/sway/commit/a3a82efbf6b5
(cherry picked from commit 4964754e0887621cd09afff61ee86c6c4c7d783b)