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

drivers/gpu/drm/drm_connector.c

@@ -1198,6 +1198,12 @@ static const u32 dp_colorspaces =
  * 	drm_connector_set_path_property(), in the case of DP MST with the
  * 	path property the MST manager created. Userspace cannot change this
  * 	property.
+ *
+ * 	In the case of DP MST, the property has the format
+ * 	``mst:<parent>-<ports>`` where ``<parent>`` is the KMS object ID of the
+ * 	parent connector and ``<ports>`` is a hyphen-separated list of DP MST
+ * 	port numbers. Note, KMS object IDs are not guaranteed to be stable
+ * 	across reboots.
  * TILE:
  * 	Connector tile group property to indicate how a set of DRM connector
  * 	compose together into one logical screen. This is used by both high-res