frame: clarify buf documentation

Mention explicitly that the array must be filled contiguously.

libavutil/frame.h

@@ -388,7 +388,9 @@ typedef struct AVFrame {
 
     /**
      * AVBuffer references backing the data for this frame. If all elements of
-     * this array are NULL, then this frame is not reference counted.
+     * this array are NULL, then this frame is not reference counted. This array
+     * must be filled contiguously -- if buf[i] is non-NULL then buf[j] must
+     * also be non-NULL for all j < i.
      *
      * There may be at most one AVBuffer per data plane, so for video this array
      * always contains all the references. For planar audio with more than