doc: try to add a link from the vf_scale sws_flags to the scaler docs

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/filters.texi

@@ -6597,6 +6597,7 @@ Default value is @samp{0}.
 @item flags
 Set libswscale scaling flags. If not explictly specified the filter
 applies a bilinear scaling algorithm.
+See the @ref{sws_flags,,,ffmpeg-scaler.texi} for the values.
 
 @item size, s
 Set the video size, the value must be a valid abbreviation or in the

doc/scaler.texi

@@ -9,6 +9,7 @@ FFmpeg tools. For programmatic use, they can be set explicitly in the
 
 @table @option
 
+@anchor{sws_flags}
 @item sws_flags
 Set the scaler flags. This is also used to set the scaling
 algorithm. Only a single algorithm should be selected.