drm/bridge: document that adding a bridge is mandatory before attach

At the moment it's not documented that you need to add a bridge before
attaching it. Clarify that.

Suggested-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/all/20250709-sophisticated-loon-of-rain-6ccdd8@houat/
Reviewed-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patch.msgid.link/20251028-b4-drm-bridge-alloc-add-before-attach-v3-3-bb8611acbbfb@bootlin.com
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>

drivers/gpu/drm/drm_bridge.c

@@ -422,6 +422,9 @@ static bool drm_bridge_is_atomic(struct drm_bridge *bridge)
  * If non-NULL the previous bridge must be already attached by a call to this
  * function.
  *
+ * The bridge to be attached must have been previously added by
+ * drm_bridge_add().
+ *
  * Note that bridges attached to encoders are auto-detached during encoder
  * cleanup in drm_encoder_cleanup(), so drm_bridge_attach() should generally
  * *not* be balanced with a drm_bridge_detach() in driver code.