doc/ffplay,ffmpeg: mention the corresponding negated option for -stats

This helps lazy users who didn't read the part about boolean options, in
case they grep for "-nostats".

-stdin and -stats are the only booleans option enabled by default. This
is a partial fix for Trac ticket #2332, a complete overhaul of boolean
options documentation is probably overkill.

doc/ffmpeg.texi

@@ -361,7 +361,8 @@ want to create filter graphs with multiple inputs and/or outputs.
 Specify the preset for matching stream(s).
 
 @item -stats (@emph{global})
-Print encoding progress/statistics. On by default.
+Print encoding progress/statistics. It is on by default, to explicitly
+disable it you need to specify @code{-nostats}.
 
 @item -progress @var{url} (@emph{global})
 Send program-friendly progress information to @var{url}.

doc/ffplay.texi

@@ -93,9 +93,13 @@ Read @var{input_file}.
 @item -pix_fmt @var{format}
 Set pixel format.
 This option has been deprecated in favor of private options, try -pixel_format.
+
 @item -stats
-Show the stream duration, the codec parameters, the current position in
-the stream and the audio/video synchronisation drift.
+Print several playback statistics, in particular show the stream
+duration, the codec parameters, the current position in the stream and
+the audio/video synchronisation drift. It is on by default, to
+explicitly disable it you need to specify @code{-nostats}.
+
 @item -bug
 Work around bugs.
 @item -fast