[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>