multimedia/wl-screenrec: document cage support after 3a87cb34b894

https://github.com/cage-kiosk/cage/commit/6f78d9d5b2a9
(cherry picked from commit 14ac189a00b57212593585f5921e751710784f23)
(cherry picked from commit 99498c3b5e3fc5a5bdf25744431e0c320efe6041)