Documentation: media: gen-errors.rst: fix confusing ENOTTY description

The text is very, very old and predates /dev/mediaX devices, so the
reference to "media device" is today very confusing.

It also says that the ioctl is not supported by the driver, but a
driver may have multiple device nodes, some support a given ioctl,
and some don't.

Simplify the description: ENOTTY means that the ioctl is not supported
by the file descriptor.

Reported-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>