fbdev: add kerneldoc do remove_conflicting_framebuffers()

Document remove_conflicting_framebuffers() behaviour.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/283fef19815b991700b56119d8d5874a2a8da44e.1535810304.git.mirq-linux@rere.qmqm.pl