[media] v4l2-framework: update documentation

The default video_device_release() callback just calls kfree to free the

allocated memory.

There is also a video_device_release_empty() function that does nothing

(is empty) and can be used if the struct is embedded and there is nothing

to do when it is released.

You should also set these fields:

- v4l2_dev: set to the v4l2_device parent device.

- v4l2_dev: must be set to the v4l2_device parent device.

- name: set to something descriptive and unique.

  If you want to have a separate priority state per (group of) device node(s),

  then you can point it to your own struct v4l2_prio_state.

- parent: you only set this if v4l2_device was registered with NULL as

- dev_parent: you only set this if v4l2_device was registered with NULL as

  the parent device struct. This only happens in cases where one hardware

  device has multiple PCI devices that all share the same v4l2_device core.

  The cx88 driver is an example of this: one core v4l2_device struct, but

  it is used by both an raw video PCI device (cx8800) and a MPEG PCI device

  (cx8802). Since the v4l2_device cannot be associated with a particular

  PCI device it is setup without a parent device. But when the struct

  video_device is setup you do know which parent PCI device to use.

  it is used by both a raw video PCI device (cx8800) and a MPEG PCI device

  (cx8802). Since the v4l2_device cannot be associated with two PCI devices

  at the same time it is setup without a parent device. But when the struct

  video_device is initialized you *do* know which parent PCI device to use and

  so you set dev_device to the correct PCI device.

- flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework

  handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct