[media] Add vfl_dir field documentation

- name: set to something descriptive and unique.

- vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0,

  so this is normally already the default), set to VFL_DIR_TX for output

  devices and VFL_DIR_M2M for mem2mem (codec) devices.

- fops: set to the v4l2_file_operations struct.

- ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance

  (highly recommended to use this and it might become compulsory in the

  future!), then set this to your v4l2_ioctl_ops struct.

  future!), then set this to your v4l2_ioctl_ops struct. The vfl_type and

  vfl_dir fields are used to disable ops that do not match the type/dir

  combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops

  are disabled for a capture device. This makes it possible to provide

  just one v4l2_ioctl_ops struct for both vbi and video nodes.

- lock: leave to NULL if you want to do all the locking in the driver.

  Otherwise you give it a pointer to a struct mutex_lock and before the