avutil/channel_layout: mention how the API user should treat channel orders it does not understand

In case new orders are added in the future, existing library users can still
use the layout simply by ignoring everything but the channel count in it, so
make this explicit.

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/channel_layout.h

@@ -290,6 +290,9 @@ typedef struct AVChannelCustom {
 typedef struct AVChannelLayout {
     /**
      * Channel order used in this layout.
+     * Any value not defined in the AVChannelOrder enum in a layout that
+     * av_channel_layout_check() doesn't reject must be treated as if it was
+     * AV_CHANNEL_ORDER_UNSPEC.
      * This is a mandatory field.
      */
     enum AVChannelOrder order;