Add docs for "{left,right,top,bottom} margin" properties.

v2:
- Mention the purpose: mitigate underscan on TVs
- Move out of analog TV section into standard props (Pekka)
- Mention HDMI AVI InfoFrames (Pekka, Ville)
Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230228123156.175973-1-contact@emersion.fr