Merge commit 'cfe64613'

* commit 'cfe64613':
  avfilter: Document avfilter_graph_alloc return value
Merged-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/avfilter.h

@@ -1245,6 +1245,8 @@ typedef struct AVFilterGraph {
 
 /**
  * Allocate a filter graph.
+ *
+ * @return the allocated filter graph on success or NULL.
  */
 AVFilterGraph *avfilter_graph_alloc(void);