[media] vidioc-enumin/output.rst: improve documentation

The V4L2_INPUT_TYPE_CAMERA and V4L2_OUTPUT_TYPE_ANALOG descriptions were
hopelessly out of date. Fix this, and also fix a few style issues in these
documents. Finally add the missing documentation for V4L2_OUTPUT_TYPE_ANALOGVGAOVERLAY
(only used by the zoran driver).

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>