doc/filters: note NetCDF dependency for sofalizer

Also add @url for link.

Signed-off-by: Lou Logan <lou@lrcd.com>

doc/filters.texi

@@ -2988,10 +2988,13 @@ silenceremove=1:5:0.02
 SOFAlizer uses head-related transfer functions (HRTFs) to create virtual
 loudspeakers around the user for binaural listening via headphones (audio
 formats up to 9 channels supported).
-The HRTFs are stored in SOFA files (see www.sofacoustics.org for a database).
+The HRTFs are stored in SOFA files (see @url{http://www.sofacoustics.org/} for a database).
 SOFAlizer is developed at the Acoustics Research Institute (ARI) of the
 Austrian Academy of Sciences.
 
+To enable compilation of this filter you need to configure FFmpeg with
+@code{--enable-netcdf}.
+
 The filter accepts the following options:
 
 @table @option