SDL_mixer: README-migration: Added some SDL2_mixer functions that were missing.

From 6269f6fef5bd81df24b3dac3b031e48ffaa7a2ed Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Mon, 1 Jun 2026 09:26:00 -0400
Subject: [PATCH] README-migration: Added some SDL2_mixer functions that were
 missing.

---
 docs/README-migration.md | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/docs/README-migration.md b/docs/README-migration.md
index 955e9bd5..c843a978 100644
--- a/docs/README-migration.md
+++ b/docs/README-migration.md
@@ -31,10 +31,10 @@ function name and a brief explanation about what to do with it.
 ## Things that are totally gone
 
 - Native MIDI has been removed (we can still play midi files like
-other file formats, just not to native midi hardware). We have
-ripped this piece out of SDL2_mixer
-  and packaged it as a separate library that can be used alongside SDL3_mixer,
-  or without SDL_mixer at all: https://github.com/libsdl-org/SDL_native_midi
+  other file formats, just not to native midi hardware). We have
+  ripped this piece out of SDL2_mixer and packaged it as a separate library
+  that can be used alongside SDL3_mixer, or without SDL_mixer at all:
+  https://github.com/libsdl-org/SDL_native_midi
 
 - Mix_GetNumTracks(), Mix_StartTrack(), Mix_ModMusicJumpToOrder(),
   Mix_SetSoundFonts(), Mix_GetSoundFonts(), Mix_EachSoundFont(),
@@ -44,6 +44,12 @@ ripped this piece out of SDL2_mixer
 
 - Mix_QuickLoad_WAV(): this was just too unsafe to offer.
 
+- Mix_SetMusicCMD(): this was messy and relied on Unix signals. If you need
+  to launch a program that makes sound outside of SDL_mixer, you can use
+  SDL3's SDL_CreateProcess() to start it in a portable way, but pausing
+  it will be difficult to do on some systems. It glib to give this advice,
+  even if the advice is good: consider just not doing this sort of thing.
+
 
 ## Including SDL3_mixer
 
@@ -412,3 +418,7 @@ can discuss it!
 - Mix_CloseAudio => MIX_DestroyMixer
 - Mix_GetError => SDL_GetError
 - Mix_SetError => SDL_SetError
+- Mix_SetMusicCMD => no equivalent in SDL3_mixer. SDL_CreateProcess can launch external programs, though.
+- Mix_GetSynchroValue => no equivalent in SDL3_mixer.
+- Mix_SetSynchroValue => no equivalent in SDL3_mixer.
+