char: misc: document behaviour of open()

an open syscall now assignes file->private_data to a pointer to the
miscdevice structure. This reminds people not to duplicate code if
they want this and not to depend on it being NULL.
Signed-off-by: NMartin Kepplinger <martink@posteo.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/misc.c

@@ -169,7 +169,9 @@ static const struct file_operations misc_fops = {
  *	the minor number requested is used.
  *
  *	The structure passed is linked into the kernel and may not be
- *	destroyed until it has been unregistered.
+ *	destroyed until it has been unregistered. By default, an open()
+ *	syscall to the device sets file->private_data to point to the
+ *	structure. Drivers don't need open in fops for this.
  *
  *	A zero is returned on success and a negative errno code for
  *	failure.