media: uapi: h264: Clarify SLICE_BASED mode

Currently, the SLICE_BASED and FRAME_BASED modes documentation
is misleading and not matching the intended use-cases.

Drop non-required fields SLICE_PARAMS 'start_byte_offset' and
DECODE_PARAMS 'num_slices' and clarify the decoding modes in the
documentation.

On SLICE_BASED mode, a single slice is expected per OUTPUT buffer,
and therefore 'start_byte_offset' is not needed (since the offset
to the slice is the start of the buffer).

This mode requires the use of CAPTURE buffer holding, and so
the number of slices shall not be required.

On FRAME_BASED mode, the devices are expected to take care of slice
parsing. Neither SLICE_PARAMS are required (and shouldn't be
exposed by frame-based drivers), nor the number of slices.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>