avfilter: Document avfilter_graph_alloc return value

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavfilter/avfilter.h

@@ -1004,6 +1004,8 @@ typedef struct AVFilterGraph {
 
 /**
  * Allocate a filter graph.
+ *
+ * @return the allocated filter graph on success or NULL.
  */
 AVFilterGraph *avfilter_graph_alloc(void);