[media] media-doc: Fix MFC display delay control doc

The V4L2_CID_MPEG_MFC51_VIDEO_DECODER_H264_DISPLAY_DELAY_ENABLE control
is a boolean but was documented as a integer. The documentation was
also slightly miss-leading.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Acked-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>