[media] doc-rst: merge v4l2-async.rst with v4l2-subdev.rst

The Async API is actually part of the v4l2 subdev.
Move its declarations to it.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/v4l2-async.rst

-V4L2 Async kAPI
-^^^^^^^^^^^^^^^
-
-.. kernel-doc:: include/media/v4l2-async.h

Documentation/media/kapi/v4l2-core.rst

@@ -5,7 +5,6 @@ Video2Linux devices
     :maxdepth: 1
 
     v4l2-framework
-    v4l2-async
     v4l2-controls
     v4l2-device
     v4l2-dv-timings

Documentation/media/kapi/v4l2-subdev.rst

@@ -266,3 +266,8 @@ V4L2 subdev kAPI
 ^^^^^^^^^^^^^^^^
 
 .. kernel-doc:: include/media/v4l2-subdev.h
+
+V4L2 subdev async kAPI
+^^^^^^^^^^^^^^^^^^^^^^
+
+.. kernel-doc:: include/media/v4l2-async.h