doc/encoders: Document png pred

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

Changelog

@@ -12,6 +12,7 @@ version <next>:
 - Animated JPEG XL encoding (via libjxl)
 - VVC in Matroska
 - CENC AV1 support in MP4 muxer
+- pngenc: set default prediction method to PAETH
 
 version 7.1:
 - Raw Captions with Time (RCWT) closed caption demuxer

doc/encoders.texi

@@ -3402,6 +3402,8 @@ Sets the compression level, from 0 to 9(default)
 Set physical density of pixels, in dots per inch, unset by default
 @item dpm @var{integer}
 Set physical density of pixels, in dots per meter, unset by default
+@item pred @var{method}
+Set prediction method (none, sub, up, avg, paeth, mixed), default is paeth
 @end table
 
 @section ProRes