[media] mc-core.rst: fix a warning about an internal routine

Fix this warning:
	Documentation/media/kapi/mc-core.rst:97: WARNING: c:func reference target not found: media_devnode_release

The media_device_release() is a function internal to media-devnode.c,
and not exported elsewhere. So, we can't cross-reference it here.
Make it explicit at the documentation.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

include/media/media-devnode.h

@@ -76,7 +76,8 @@ struct media_file_operations {
  * @parent:	parent device
  * @minor:	device node minor number
  * @flags:	flags, combination of the ``MEDIA_FLAG_*`` constants
- * @release:	release callback called at the end of media_devnode_release()
+ * @release:	release callback called at the end of ``media_devnode_release()``
+ *		routine at media-device.c.
  *
  * This structure represents a media-related device node.
  *