output: add comment about needs_frame in wlr_output_schedule_frame
output: add comment about needs_frame in wlr_output_schedule_frame

Add a comment to not forget why this call is necessary.

References: https://github.com/swaywm/wlroots/pull/2053

ascent12/wlroots,swaywm/wlroots,SirCmpwn/wlroots,ascent12/wlroots,swaywm/wlroots