lavfi: document assumptions about the input and output labels of a filter graph description

doc/filters.texi

@@ -195,6 +195,10 @@ instance two input pads. The first output pad of split is labelled
 output pad of split is linked to the second input pad of overlay,
 which are both unlabelled.
 
+In a filter description, if the input label of the first filter is not
+specified, "in" is assumed; if the output label of the last filter is not
+specified, "out" is assumed.
+
 In a complete filterchain all the unlabelled filter input and output
 pads must be connected. A filtergraph is considered valid if all the
 filter input and output pads of all the filterchains are connected.

libavfilter/avfilter.h

@@ -1419,6 +1419,10 @@ int avfilter_graph_parse(AVFilterGraph *graph, const char *filters,
 /**
  * Add a graph described by a string to a graph.
  *
+ * In the graph filters description, if the input label of the first
+ * filter is not specified, "in" is assumed; if the output label of
+ * the last filter is not specified, "out" is assumed.
+ *
  * @param graph   the filter graph where to link the parsed graph context
  * @param filters string to be parsed
  * @param inputs  pointer to a linked list to the inputs of the graph, may be NULL.