[media] DocBook media: improve control section

Improve the control section:

- Clarify the handling of private controls
- Explain the V4L2_CTRL_FLAG_INACTIVE flag
- Remove obsolete text regarding missing control event (we have them
  today) and the incorrect V4L2_CTRL_FLAG_DISABLED reference.
- Add a code example on how to enumerate over user controls.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>