options: explicitly deprecate --ad-spdif-dtshd

Has been less formally deprecated for a longer time.

DOCS/interface-changes.rst

@@ -46,6 +46,8 @@ Interface changes
     - change how spdif codecs are selected. You can't enable spdif passthrough
       with --ad anymore. This was deprecated; use --audio-spdif instead.
     - deprecate the "family" selection with --ad/--vd
+    - explicitly mark --ad-spdif-dtshd as deprecated (it was done so a long time
+      ago, but it didn't complain when using the option)
  --- mpv 0.22.0 ---
     - the "audio-device-list" property now sets empty device description to the
       device name as a fallback

options/options.c

@@ -426,7 +426,8 @@ const m_option_t mp_opts[] = {
 
     OPT_STRING("audio-spdif", audio_spdif, 0),
 
-    OPT_FLAG("ad-spdif-dtshd", dtshd, 0),
+    OPT_FLAG("ad-spdif-dtshd", dtshd, 0,
+             .deprecation_message = "use --audio-spdif instead"),
 
     OPT_CHOICE_C("hwdec", hwdec_api, 0, mp_hwdec_names),
     OPT_STRING("hwdec-codecs", hwdec_codecs, 0),