drm/doc: describe PATH format for DP MST

This is already uAPI, xserver parses it. It's useful to document
since user-space might want to lookup the parent connector.

Additionally, people (me included) have misunderstood the PATH
property for being stable across reboots, but since a KMS object
ID is baked in there that's not the case. So PATH shouldn't be
used as-is in config files and such.

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20231023203629.198109-1-contact@emersion.fr