drm: document that user-space should force-probe connectors

It seems like we can't have nice things, so let's just document the
disappointing behaviour instead.

The previous version assumed the kernel would perform the probing work
when appropriate, however this is not the case today. Update the
documentation to reflect reality.

v2:

- Improve commit message to explain why this change is made (Pekka)
- Keep the bit about flickering (Daniel)
- Explain when user-space should force-probe, and when it shouldn't (Daniel)

Signed-off-by: Simon Ser <contact@emersion.fr>
Fixes: 2ac5ef3b ("drm: document drm_mode_get_connector")
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/AxqLnTAsFCRishOVB5CLsqIesmrMrm7oytnOVB7oPA@cp7-web-043.plabs.ch