avfiltergraph: clarify doxy for avfilter_graph_parse() open_inputs/outputs params

Make it clear that open_inputs/open_outputs are updated only if non-NULL.

libavfilter/avfiltergraph.h

@@ -124,12 +124,12 @@ void avfilter_inout_free(AVFilterInOut **inout);
  *
  * @param graph   the filter graph where to link the parsed graph context
  * @param filters string to be parsed
- * @param inputs  linked list to the inputs of the graph, may be NULL.
- *                It is updated to contain the list of open inputs after the parsing,
- *                should be freed with avfilter_inout_free().
- * @param outputs linked list to the outputs of the graph, may be NULL.
- *                It is updated to contain the list of open outputs after the parsing,
- *                should be freed with avfilter_inout_free().
+ * @param inputs  pointer to a linked list to the inputs of the graph, may be NULL.
+ *                If non-NULL, *inputs is updated to contain the list of open inputs
+ *                after the parsing, should be freed with avfilter_inout_free().
+ * @param outputs pointer to a linked list to the outputs of the graph, may be NULL.
+ *                If non-NULL, *outputs is updated to contain the list of open outputs
+ *                after the parsing, should be freed with avfilter_inout_free().
  * @return zero on success, a negative AVERROR code on error
  */
 int avfilter_graph_parse(AVFilterGraph *graph, const char *filters,