media: vidioc-g-ext-ctrls.rst: reformat tables and clarify which vs ctrl_class

media: vidioc-g-ext-ctrls.rst: reformat tables and clarify which vs ctrl_class

The VIDIOC_G/S/TRY_EXT_CTRLS documentation has large explanatory texts for
some of the fields in a table. This makes it hard to read. Move those text
to a new cell spanning the whole width of the table, similar to what was
done for struct v4l2_pix_format. This makes it much more readable.

Also move the 'ctrl_class' description to below the 'which' description and
just mention that it is deprecated and that 'which' should be used instead.

Finally remove 'note::' for the V4L2_CTRL_WHICH_DEF_VAL description. It
doesn't have to be marked as a note, it's just a simple paragraph.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>