[media] doc-rst: fix htmldocs build warnings

Fix those warnings:
	Documentation/output/videodev2.h.rst:6: WARNING: undefined label: vidioc_unsubscribe_event (if the link has no caption the label must precede a section header)
	Documentation/media/uapi/v4l/dev-overlay.rst:248: WARNING: Title underline too short.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/uapi/v4l/dev-overlay.rst

@@ -245,7 +245,7 @@ exceeded are undefined. [#f3]_
 .. _v4l2-clip:
 
 struct v4l2_clip [#f4]_
----------------------
+-----------------------
 
 ``struct v4l2_rect c``
     Coordinates of the clipping rectangle, relative to the top, left

Documentation/media/uapi/v4l/vidioc-subscribe-event.rst

@@ -1,6 +1,7 @@
 .. -*- coding: utf-8; mode: rst -*-
 
 .. _VIDIOC_SUBSCRIBE_EVENT:
+.. _VIDIOC_UNSUBSCRIBE_EVENT:
 
 ******************************************************
 ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT
@@ -54,8 +55,8 @@ using the :ref:`VIDIOC_DQEVENT` ioctl.
        -  Type of the event, see :ref:`event-type`.
 
 	  .. note:: ``V4L2_EVENT_ALL`` can be used with
-	     :ref:`VIDIOC_UNSUBSCRIBE_EVENT` for unsubscribing all events
-	     at once.
+	     :ref:`VIDIOC_UNSUBSCRIBE_EVENT <VIDIOC_SUBSCRIBE_EVENT>` for
+	     unsubscribing all events at once.
 
     -  .. row 2
 

Documentation/media/videodev2.h.rst.exceptions

@@ -531,6 +531,5 @@ replace ioctl VIDIOC_TRY_ENCODER_CMD vidioc_encoder_cmd
 replace ioctl VIDIOC_TRY_EXT_CTRLS vidioc_g_ext_ctrls
 replace ioctl VIDIOC_TRY_FMT vidioc_g_fmt
 replace ioctl VIDIOC_STREAMOFF vidioc_streamon
-replace ioctl VIDIOC_UNSUBSCRIBE_EVENT vidioc_subscribe_event
 replace ioctl VIDIOC_QUERY_EXT_CTRL vidioc_queryctrl
 replace ioctl VIDIOC_QUERYMENU vidioc_queryctrl