Mention explicitly that the array must be filled contiguously.
/**
* 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