examples/filtering: extend comments about setting the filter graph endpoints

That part is a constant source of confusions for readers, this should
help clarify the apparently inverted logic.

doc/examples/filtering_audio.c

@@ -145,12 +145,28 @@ static int init_filters(const char *filters_descr)
         goto end;
     }
 
-    /* Endpoints for the filter graph. */
+    /*
+     * Set the endpoints for the filter graph. The filter_graph will
+     * be linked to the graph described by filters_descr.
+     */
+
+    /*
+     * The buffer source output must be connected to the input pad of
+     * the first filter described by filters_descr; since the first
+     * filter input label is not specified, it is set to "in" by
+     * default.
+     */
     outputs->name       = av_strdup("in");
     outputs->filter_ctx = buffersrc_ctx;
     outputs->pad_idx    = 0;
     outputs->next       = NULL;
 
+    /*
+     * The buffer sink input must be connected to the output pad of
+     * the last filter described by filters_descr; since the last
+     * filter output label is not specified, it is set to "out" by
+     * default.
+     */
     inputs->name       = av_strdup("out");
     inputs->filter_ctx = buffersink_ctx;
     inputs->pad_idx    = 0;

doc/examples/filtering_video.c

@@ -128,12 +128,28 @@ static int init_filters(const char *filters_descr)
         goto end;
     }
 
-    /* Endpoints for the filter graph. */
+    /*
+     * Set the endpoints for the filter graph. The filter_graph will
+     * be linked to the graph described by filters_descr.
+     */
+
+    /*
+     * The buffer source output must be connected to the input pad of
+     * the first filter described by filters_descr; since the first
+     * filter input label is not specified, it is set to "in" by
+     * default.
+     */
     outputs->name       = av_strdup("in");
     outputs->filter_ctx = buffersrc_ctx;
     outputs->pad_idx    = 0;
     outputs->next       = NULL;
 
+    /*
+     * The buffer sink input must be connected to the output pad of
+     * the last filter described by filters_descr; since the last
+     * filter output label is not specified, it is set to "out" by
+     * default.
+     */
     inputs->name       = av_strdup("out");
     inputs->filter_ctx = buffersink_ctx;
     inputs->pad_idx    = 0;