doc/filters: Add stats_version documentation to PSNR filter

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/filters.texi

@@ -10692,13 +10692,31 @@ The description of the accepted parameters follows.
 If specified the filter will use the named file to save the PSNR of
 each individual frame. When filename equals "-" the data is sent to
 standard output.
+
+@item stats_version
+Specifies which version of the stats file format to use. Details of
+each format are written below.
+Default value is 1.
 @end table
 
 The file printed if @var{stats_file} is selected, contains a sequence of
 key/value pairs of the form @var{key}:@var{value} for each compared
 couple of frames.
 
-A description of each shown parameter follows:
+If a @var{stats_version} greater than 1 is specified, a header line precedes
+the list of per-frame-pair stats, with key value pairs following the frame
+format with the following parameters:
+
+@table @option
+@item psnr_log_version
+The version of the log file format. Will match @var{stats_version}.
+
+@item fields
+A comma separated list of the per-frame-pair parameters included in
+the log.
+@end table
+
+A description of each shown per-frame-pair parameter follows:
 
 @table @option
 @item n