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 <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>
Reviewed-by: Nicolas Dufresne <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>

Documentation/userspace-api/media/gen-errors.rst

···
 
     -  -  ``ENOTTY``
 
-       -  The ioctl is not supported by the driver, actually meaning that
-	  the required functionality is not available, or the file
-	  descriptor is not for a media device.
+       -  The ioctl is not supported by the file descriptor.
 
     -  -  ``ENOSPC``