fftools/ffmpeg_opt: Document VAAPI -device usage for DirectX Adapter

Initial review at https://github.com/intel-media-ci/ffmpeg/pull/619/

Signed-off-by: Sil Vilerino <sivileri@microsoft.com>
Reviewed-by: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Reviewed-by: Wu, Tong1 <tong1.wu@intel.com>

doc/ffmpeg.texi

@@ -1169,9 +1169,10 @@ Choose the first device and use the primary device context.
 @var{device} is the number of the Direct3D 11 display adapter.
 
 @item vaapi
-@var{device} is either an X11 display name or a DRM render node.
+@var{device} is either an X11 display name, a DRM render node or a DirectX adapter index.
 If not specified, it will attempt to open the default X11 display (@emph{$DISPLAY})
-and then the first DRM render node (@emph{/dev/dri/renderD128}).
+and then the first DRM render node (@emph{/dev/dri/renderD128}), or the default
+DirectX adapter on Windows.
 
 @item vdpau
 @var{device} is an X11 display name.

fftools/ffmpeg_opt.c

@@ -1767,7 +1767,7 @@ const OptionDef options[] = {
 
 #if CONFIG_VAAPI
     { "vaapi_device", HAS_ARG | OPT_EXPERT, { .func_arg = opt_vaapi_device },
-        "set VAAPI hardware device (DRM path or X11 display name)", "device" },
+        "set VAAPI hardware device (DirectX adapter index, DRM path or X11 display name)", "device" },
 #endif
 
 #if CONFIG_QSV