doc/filters: add entry for mestimate_d3d12

Add documentation for the filter "mestimate_d3d12" to the filters documentation.

This is the follow-up for commit https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/21217/commits

Signed-off-by: younengxiao <steven.xiao@amd.com>

doc/filters.texi

@@ -18096,6 +18096,35 @@ Macroblock size. Default @code{16}.
 Search parameter. Default @code{7}.
 @end table
 
+@section mestimate_d3d12
+
+Estimate and export motion vectors using D3D12 hardware-accelerated motion estimation.
+This filter uses GPU hardware motion estimation capabilities available in
+DirectX 12 Video APIs to achieve significant performance improvements
+over the software-based @code{mestimate} filter.
+
+Motion vectors are stored in frame side data to be used by other filters.
+
+This filter requires the input to be in @code{d3d12} hardware pixel format.
+The filter uses quarter-pixel precision for motion vector estimation.
+
+This filter accepts the following options:
+@table @option
+@item mb_size
+Macroblock size. Only @code{8} and @code{16} are supported.
+Default @code{16}.
+@end table
+
+@subsection Examples
+
+Estimate motion vectors using D3D12 hardware acceleration with 16x16 blocks,
+and visualize them:
+@example
+ffmpeg -hwaccel d3d12va -hwaccel_output_format d3d12 -i input.mp4 \
+        -vf mestimate_d3d12=mb_size=16,hwdownload,format=nv12,codecview=mv=pf \
+        -c:v libx264 -preset fast -b:v 5M output.mp4
+@end example
+
 @section midequalizer
 
 Apply Midway Image Equalization effect using two video streams.