update doc on custom mode

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>

include/opus_custom.h

@@ -104,7 +104,8 @@ typedef struct OpusCustomDecoder OpusCustomDecoder;
 /** The mode contains all the information necessary to create an
     encoder. Both the encoder and decoder need to be initialized
     with exactly the same mode, otherwise the output will be
-    corrupted.
+    corrupted. The mode MUST NOT BE DESTROYED until the encoders and
+    decoders that use it are destroyed as well.
    @brief Mode configuration
  */
 typedef struct OpusCustomMode OpusCustomMode;