[media] v4l2-async: document the remaining stuff

There are one enum and 4 functions undocumented there.
Document them. That will fix the broken links at the
v4l2-subdev.rst file.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

include/media/v4l2-async.h

@@ -23,6 +23,19 @@ struct v4l2_async_notifier;
 /* A random max subdevice number, used to allocate an array on stack */
 #define V4L2_MAX_SUBDEVS 128U
 
+/**
+ * enum v4l2_async_match_type - type of asynchronous subdevice logic to be used
+ *	in order to identify a match
+ *
+ * @V4L2_ASYNC_MATCH_CUSTOM: Match will use the logic provided by &struct
+ * 	v4l2_async_subdev.match ops
+ * @V4L2_ASYNC_MATCH_DEVNAME: Match will use the device name
+ * @V4L2_ASYNC_MATCH_I2C: Match will check for I2C adapter ID and address
+ * @V4L2_ASYNC_MATCH_OF: Match will use OF node
+ *
+ * This enum is used by the asyncrhronous sub-device logic to define the
+ * algorithm that will be used to match an asynchronous device.
+ */
 enum v4l2_async_match_type {
 	V4L2_ASYNC_MATCH_CUSTOM,
 	V4L2_ASYNC_MATCH_DEVNAME,
@@ -91,9 +104,35 @@ struct v4l2_async_notifier {
 		       struct v4l2_async_subdev *asd);
 };
 
+/**
+ * v4l2_async_notifier_register - registers a subdevice asynchronous notifier
+ *
+ * @v4l2_dev: pointer to &struct v4l2_device
+ * @notifier: pointer to &struct v4l2_async_notifier
+ */
 int v4l2_async_notifier_register(struct v4l2_device *v4l2_dev,
 				 struct v4l2_async_notifier *notifier);
+
+/**
+ * v4l2_async_notifier_unregister - unregisters a subdevice asynchronous notifier
+ *
+ * @notifier: pointer to &struct v4l2_async_notifier
+ */
 void v4l2_async_notifier_unregister(struct v4l2_async_notifier *notifier);
+
+/**
+ * v4l2_async_register_subdev - registers a sub-device to the asynchronous
+ * 	subdevice framework
+ *
+ * @sd: pointer to &struct v4l2_subdev
+ */
 int v4l2_async_register_subdev(struct v4l2_subdev *sd);
+
+/**
+ * v4l2_async_unregister_subdev - unregisters a sub-device to the asynchronous
+ * 	subdevice framework
+ *
+ * @sd: pointer to &struct v4l2_subdev
+ */
 void v4l2_async_unregister_subdev(struct v4l2_subdev *sd);
 #endif