char: misc: document behaviour of open()

an open syscall now assignes file->private_data to a pointer to the
miscdevice structure. This reminds driver developers not to duplicate
code if they need this.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>