[media] v4l2-framework.txt: document v4l2_dont_use_cmd

Do not use .ioctl! This is deprecated and will go away in the future.

In some cases you want to tell the core that a function you had specified in

your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this

function before video_device_register is called:

void v4l2_dont_use_cmd(struct video_device *vdev, unsigned int cmd);

This tends to be needed if based on external factors (e.g. which card is

being used) you want to turns off certain features in v4l2_ioctl_ops without

having to make a new struct.

The v4l2_file_operations struct is a subset of file_operations. The main

difference is that the inode argument is omitted since it is never used.