doc/filters: add thumbnail_cuda entry

Also update thumbnail_cuda filter description.

doc/filters.texi

@@ -26917,6 +26917,34 @@ scale_cuda=passthrough=0
 @end example
 @end itemize
 
+@subsection thumbnail_cuda
+
+Select the most representative frame in a given sequence of consecutive frames using CUDA.
+
+The filter accepts the following options:
+
+@table @option
+@item n
+Set the frames batch size to analyze; in a set of @var{n} frames, the filter
+will pick one of them, and then handle the next batch of @var{n} frames until
+the end. Default is @code{100}.
+@end table
+
+Since the filter keeps track of the whole frames sequence, a bigger @var{n}
+value will result in a higher memory usage, so a high value is not recommended.
+
+@subsubsection Example
+
+@itemize
+
+@item
+Thumbnails are extracted from every @var{n}=150-frame batch, selecting one per batch. Chosen frames are then scaled with @ref{scale_cuda}.
+@example
+./ffmpeg  -hwaccel cuda -hwaccel_output_format cuda  -i ./input.mp4 -vf "thumbnail_cuda=150,scale_cuda=1920:1080,hwdownload,format=nv12" ./output/out%03d.png
+@end example
+
+@end itemize
+
 @subsection yadif_cuda
 
 Deinterlace the input video using the @ref{yadif} algorithm, but implemented

libavfilter/vf_thumbnail_cuda.c

@@ -438,7 +438,7 @@ static const AVFilterPad thumbnail_cuda_outputs[] = {
 
 const FFFilter ff_vf_thumbnail_cuda = {
     .p.name        = "thumbnail_cuda",
-    .p.description = NULL_IF_CONFIG_SMALL("Select the most representative frame in a given sequence of consecutive frames."),
+    .p.description = NULL_IF_CONFIG_SMALL("Select the most representative frame in a given sequence of consecutive frames using CUDA."),
     .p.priv_class  = &thumbnail_cuda_class,
     .priv_size     = sizeof(ThumbnailCudaContext),
     .init          = init,