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: 2ac5ef3b23 ("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

include/uapi/drm/drm_mode.h

@@ -414,15 +414,12 @@ enum drm_mode_subconnector {
  *
  * If the @count_modes field is set to zero, the kernel will perform a forced
  * probe on the connector to refresh the connector status, modes and EDID.
- * A forced-probe can be slow and the ioctl will block. A force-probe can cause
- * flickering and temporary freezes, so it should not be performed
- * automatically.
+ * A forced-probe can be slow, might cause flickering and the ioctl will block.
  *
- * User-space shouldn't need to force-probe connectors in general: the kernel
- * will automatically take care of probing connectors that don't support
- * hot-plug detection when appropriate. However, user-space may force-probe
- * connectors on user request (e.g. clicking a "Scan connectors" button, or
- * opening a UI to manage screens).
+ * User-space needs to force-probe connectors to ensure their metadata is
+ * up-to-date at startup and after receiving a hot-plug event. User-space
+ * may perform a forced-probe when the user explicitly requests it. User-space
+ * shouldn't perform a forced-probe in other situations.
  */
 struct drm_mode_get_connector {
 	/** @encoders_ptr: Pointer to ``__u32`` array of object IDs. */