media: docs: uAPI: Clarify error documentation for invalid 'which' value

Invalid values for the 'which' field of structures passed to multiple
subdev ioctls result in an EINVAL error being returned. The
documentation of the corresponding ioctls indicates this with sentences
such as the following:

    the ``which`` field references a non-existing format

This is confusing. Clarify the documentation.

Suggested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>