media: uapi: h264: clarify dec_ref_pic_marking_bit_size fields

Since dec_ref_pic_marking_bit_size is not a syntax element
itself, explicitly state that this is the size in bits of
the dec_ref_pic_marking() syntax element contained in the
slice.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>