ffplay: extend documentation for the -vf option

In particular, make it consistent with the ffmpeg docs.

doc/ffplay.texi

@@ -74,10 +74,15 @@ You can interactively cycle through the available show modes by
 pressing the key @key{w}.
 
 @item -vf @var{filter_graph}
+Create the filter graph specified by @var{filter_graph} and use it to
+filter the video stream.
+
 @var{filter_graph} is a description of the filter graph to apply to
-the input video.
-Use the option "-filters" to show all the available filters (including
-also sources and sinks).
+the stream, and must have a single video input and a single video
+output. In the filter graph, the input is associated to the label
+@code{in}, and the output to the label @code{out}. See the
+ffmpeg-filters manual for more information about the filtergraph
+syntax.
 
 @item -i @var{input_file}
 Read @var{input_file}.

ffplay.c

@@ -3241,7 +3241,7 @@ static const OptionDef options[] = {
     { "infbuf", OPT_BOOL | OPT_EXPERT, { &infinite_buffer }, "don't limit the input buffer size (useful with realtime streams)", "" },
     { "window_title", OPT_STRING | HAS_ARG, { &window_title }, "set window title", "window title" },
 #if CONFIG_AVFILTER
-    { "vf", OPT_STRING | HAS_ARG, { &vfilters }, "video filters", "filter list" },
+    { "vf", OPT_STRING | HAS_ARG, { &vfilters }, "set video filters", "filter_graph" },
 #endif
     { "rdftspeed", OPT_INT | HAS_ARG| OPT_AUDIO | OPT_EXPERT, { &rdftspeed }, "rdft speed", "msecs" },
     { "showmode", HAS_ARG, { .func_arg = opt_show_mode}, "select show mode (0 = video, 1 = waves, 2 = RDFT)", "mode" },