[media] DocBook/media: clarify control documentation

- Add missing V4L2_CTRL_TYPE_U32 documentation
- Remove 'which are actually different on the hardware' sentence which
  is confusing. I think the idea was to let the user know that the step can
  be different for different hardware, but that's obvious because otherwise
  you wouldn't need to specify the step value.
- Clarify that V4L2_CTRL_COMPOUND_TYPES also applies to arrays.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>