drm: document how user-space should use link-status

Describe what a "BAD" link-status means for user-space and how it should
handle it. The logic described has been implemented in igt [1].

v2:

- Change wording to avoid "enabled" (Daniel)
- Add paragraph about multiple connectors sharing the same CRTC (Pekka)
- Add paragraph about performing an atomic commit on a connector without
  updating the link-status property (Daniel)

v3:

- Fix description of what happens when link-status isn't reset to
  "GOOD", and when link-status is reset without ALLOW_MODESET (Daniel,
  Ville)
- Changing link-status to "BAD" is a no-op
- Clearly state that "BAD" means black screen (Manasi)

[1]: https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/commit/fbe61f529737191d0920521946a575bd55f00fbe



Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/kFylMmeRMGJk-oi8f-Td8A7GNa1C-GsK23-vjKg77VhWfwpkLJg7QxFlQ_g9KdVxZiyWl9eJWpUGa5PYasR9YcyvIbuBmHVfKeHb4rH0yTM=@emersion.fr