doc/encoders: document libjxl encoder options

Add more detailed documenation for the libjxl encoder
wrapper than is present currently inside libavcodec.

doc/encoders.texi

@@ -1809,6 +1809,41 @@ by ":". See the SVT-AV1 encoder user guide for a list of accepted parameters.
 
 @end table
 
+@section libjxl
+
+libjxl JPEG XL encoder wrapper.
+
+Requires the presence of the libjxl headers and library during
+configuration. You need to explicitly configure the build with
+@code{--enable-libjxl}.
+
+@subsection Options
+
+The libjxl wrapper supports the following options:
+
+@table @option
+
+@item distance
+Set the target Butteraugli distance. This is a quality setting: lower
+distance yields higher quality, with distance=1.0 roughly comparable to
+libjpeg Quality 90 for photographic content. Setting distance=0.0 yields
+true lossless encoding. Valid values range between 0.0 and 15.0, and sane
+values rarely exceed 5.0. Setting distance=0.1 usually attains
+transparency for most input. The default is 1.0.
+
+@item effort
+Set the encoding effort used. Higher effort values produce more consistent
+quality and usually produces a better quality/bpp curve, at the cost of
+more CPU time required. Valid values range from 1 to 9, and the default is 7.
+
+@item modular
+Force the encoder to use Modular mode instead of choosing automatically. The
+default is to use VarDCT for lossy encoding and Modular for lossless. VarDCT
+is generally superior to Modular for lossy encoding but does not support
+lossless encoding.
+
+@end table
+
 @section libkvazaar
 
 Kvazaar H.265/HEVC encoder.