avutil/channel_layout: move and improve the comment about unknown orders

Don't place it as doxy specific for the order field, and generalize it both to
also cover already defined orders and to not make it seem like the user is
required to handle a layout they don't fully support or understand.

Signed-off-by: James Almer <jamrial@gmail.com>

libavutil/channel_layout.h

@@ -266,6 +266,9 @@ typedef struct AVChannelCustom {
  * A channel layout here is defined as a set of channels ordered in a specific
  * way (unless the channel order is AV_CHANNEL_ORDER_UNSPEC, in which case an
  * AVChannelLayout carries only the channel count).
+ * All orders may be treated as if they were AV_CHANNEL_ORDER_UNSPEC by
+ * ignoring everything but the channel count, as long as av_channel_layout_check()
+ * considers they are valid.
  *
  * Unlike most structures in Libav, sizeof(AVChannelLayout) is a part of the
  * public ABI and may be used by the caller. E.g. it may be allocated on stack
@@ -290,9 +293,6 @@ 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;